Electrical & Off-Grid Power

I ran three campervans without proper battery monitoring systems. The first had nothing but a basic voltmeter that told me almost nothing useful. The second had a fancy digital display that was essentially lying to me about battery state. The third had a “percentage” display that was so inaccurate I might as well have been reading tea leaves.

Between those three vans, I underestimated my battery capacity twice (ran power too low, damaged batteries), overestimated it countless times (thought I had power when I didn’t), and made decisions based on voltage readings that were fundamentally misleading. Cost me two batteries (£330 total), loads of frustration, and probably 50+ hours of my life worrying about power.

After 30 years as a maintenance manager, you’d think I’d understand the importance of accurate measurement. I do – in buildings. But vehicle electrical systems are different. Batteries behave in ways that aren’t intuitive. Voltage is a terrible indicator of state of charge. And without proper monitoring, you’re basically flying blind.

This guide is everything I wish I’d known before I installed my first leisure battery. Not the theory. Not the perfect setups. The actual reality of managing power in a van, what monitoring genuinely helps, and how to stop guessing and start knowing what’s actually happening with your electrical system.

This guide will explain the importance of Battery Monitoring Systems in campervans and how they can transform your power management experience.

Why I Wasted 3 Years Without Proper Monitoring

Van #1 (2018-2019): The Voltmeter That Told Me Nothing

Monitoring setup: Basic analog voltmeter (£6 from eBay)

What it showed: Voltage. Just voltage. 12.8V… 12.4V… 11.9V…

What I thought that meant:

12.8V = Battery full
12.4V = Battery half full
11.9V = Battery nearly empty

Reality: This is completely wrong for lithium batteries, and misleading for AGM.

What actually happened:

Day 1 camping: Voltage showed 12.7V. “Great, battery’s full!” Used lights, fridge, laptop all evening. Next morning: 12.3V. “Still got loads of power.” Used more power. That evening: 12.0V. “Getting low.” Started diesel heater overnight. Next morning: Battery dead. 11.2V. Wouldn’t even run the water pump.

Actual capacity used: Started with 110Ah battery at 90% (99Ah). Used approximately 85Ah over 24 hours. Battery was at 14Ah remaining (13% full) when I thought I had “loads of power.”

Why voltage is useless:

AGM voltage curve is relatively flat between 80% and 20% state of charge. Voltage drops from 12.6V to 12.1V across 60% of capacity. You can’t tell 70% full from 30% full by voltage alone.

Cost: Ran AGM too low repeatedly. Battery died after 4 months (normal lifespan: 2-4 years). £95 wasted.

Van #2 (2020-2021): The Display That Lied

Monitoring setup: “Smart” battery monitor with percentage display (£35)

What it showed: Voltage and a “percentage” (12.4V – 68%)

What I thought: Finally, proper monitoring! I can see actual state of charge!

Reality: The “percentage” was calculated from voltage using a lookup table. Not actual amp-hour counting. Just as useless as the voltmeter but with false confidence.

What actually happened:

Trip to Scotland. Started at “100%” (actually 95%). Used power normally. Display showed: 82%… 71%… 65%. “Plenty of power left.” Then suddenly: 42%… 28%… 15% in the space of 3 hours. What?

Voltage under load dropped faster than expected. Display panicked. I panicked. Drove for 2 hours to recharge (unnecessary – battery was actually fine, just under load).

The problem: Voltage-based “percentages” don’t account for:

Load (heavy load drops voltage temporarily)
Temperature (cold batteries show lower voltage)
Battery type differences
Actual amp-hours consumed

It was guessing. Badly.

Cost: £35 for a monitor that was barely better than a voltmeter. Plus stress and unnecessary driving.

Van #3 (2022-2023): Better Battery, Still Guessing

Monitoring setup: Same voltage-based monitor

Battery: Upgraded to 200Ah AGM (£420)

Problem: With a bigger battery, the guessing game was even worse. I had no idea if I’d used 50Ah or 100Ah. The voltage-based percentage was even less accurate with the larger capacity.

Result: Either drove to recharge too early (wasting time) or ran battery too low (damaging it).

After 18 months, battery capacity had dropped to about 140Ah (from 200Ah). Killed it through poor management and deep discharging without realizing.

Cost: Battery degraded faster than it should have. Lost 2 years of life. Value loss: £150-180.

Van #4 (2023-present): Finally Got It Right

Monitoring setup: Victron BMV-712 battery monitor (£185)

What it does: Actually counts amp-hours in and out. Shows:

Real state of charge (based on actual Ah consumed, not voltage)
Current draw/charge (in amps)
Power (in watts)
Time remaining at current draw
Historical data
Bluetooth to phone app

Result: Complete transformation. I finally know what’s actually happening.

Example from last week:

Morning: 87% (91Ah remaining of 105Ah) Turned on fridge: Drawing 4A Made coffee (inverter + laptop): Drawing 6A total Total draw: 10A Time remaining: 9.1 hours at current draw

Actually useful information.

Used 22Ah during the day. Evening: 66% (69Ah remaining). Knew exactly how much capacity I had. Planned accordingly. Drove next day to recharge (or could have relied on solar – summer).

No guessing. No stress. No damaged batteries.

Cost: £185. Should have bought it for Van #1. Would have saved me £330 in damaged batteries plus countless hours of worry.

Understanding Battery Monitoring Systems and State of Charge (Why Voltage Lies)

This is the fundamental problem everyone gets wrong.

Voltage vs State of Charge: The Misleading Relationship

AGM battery voltage curve:

State of Charge	Resting Voltage	Under 10A Load	Under 30A Load
100%	12.7V	12.5V	12.2V
80%	12.5V	12.3V	11.9V
60%	12.3V	12.1V	11.7V
40%	12.1V	11.9V	11.4V
20%	11.9V	11.6V	11.1V
0%	11.7V	11.3V	10.8V

Notice: Under load, voltage drops significantly. 12.2V under load could be 100% full or 80% full. You can’t tell.

Lithium battery voltage curve (even worse):

State of Charge	Resting Voltage	Under 10A Load	Under 30A Load
100%	13.4V	13.3V	13.2V
80%	13.3V	13.2V	13.1V
60%	13.2V	13.1V	13.0V
40%	13.0V	12.9V	12.8V
20%	12.9V	12.7V	12.6V
10%	12.5V	12.3V	12.0V

Notice: Voltage is incredibly flat from 100% to 40%. 13.2V could be 100% or 60% full. Completely useless for determining state.

This is why I had no idea what was happening in Van #1-3.

What You Actually Need to Know

1. Amp-hours consumed:

Started with: 105Ah
Consumed: 22Ah
Remaining: 83Ah
State of charge: 79%

This is real information.

2. Current draw:

Fridge: 4A
Lights: 1A
Total: 5A

This tells you what’s using power right now.

3. Time remaining:

83Ah remaining ÷ 5A draw = 16.6 hours

This tells you how long you can continue at current consumption.

4. Historical data:

Yesterday consumed: 28Ah
This week average: 25Ah/day
Solar generated today: 42Ah

This tells you patterns and trends.

None of this is available from a voltmeter.

How Proper Monitoring Actually Works (Shunt-Based Systems)

The shunt method:

A “shunt” (precision resistor) is installed in the negative cable between battery and loads
All current flows through the shunt
Monitor measures voltage drop across shunt
Calculates current using Ohm’s law (V = IR)
Integrates current over time to count amp-hours
Tracks state of charge by counting Ah in (charging) and Ah out (discharge)

Example:

Morning: Battery at 100% (105Ah)

Fridge draws 4A for 12 hours = 48Ah consumed
Lights draw 1A for 4 hours = 4Ah consumed
Laptop charging draws 6A for 2 hours = 12Ah consumed
Total consumed: 64Ah

Evening: Battery at 39% (41Ah remaining)

Next day:

Solar charges at 10A for 5 hours = 50Ah added
Battery now: 87% (91Ah)

Accurate. Real. Useful.

This is what I have now. It’s transformed how I manage power.

Battery Monitor Types: What’s Actually Available

Type 1: Basic Voltmeter (£5-£15)

What it is: Simple voltage display

What it shows: 12.4V

Pros:

Cheap (£5-15)
Simple
Never breaks

Cons:

Voltage is useless for state of charge
No current information
No historical data
Basically decorative

My experience: Van #1. Useless. Don’t buy.

Verdict: Save your £10. Buy nothing instead.

Type 2: Voltage-Based “Smart” Monitors (£25-£50)

What it is: Voltage monitor with percentage calculated from voltage lookup table

What it shows: 12.4V – 68%

Pros:

Cheap (£25-50)
Shows “percentage” (comforting but inaccurate)
Easy to install

Cons:

Percentage is guessed from voltage (wrong)
No current measurement
No amp-hour counting
False sense of accuracy

My experience: Van #2-3. Marginally better than voltmeter but still fundamentally flawed.

Verdict: Don’t bother. You’re paying £30 for a fancy voltmeter that guesses.

Type 3: Shunt-Based Battery Monitors (£60-£200)

What it is: Proper monitoring with shunt to measure current and count amp-hours

What it shows:

Voltage: 13.2V
Current: -8A (discharging)
Power: -96W
Consumed: 28Ah
State of charge: 73%
Time remaining: 9.4 hours

Pros:

Accurate state of charge (actual Ah counting)
Current and power measurement
Time remaining estimates
Historical data
Programmable for battery type
Bluetooth monitoring (on better models)

Cons:

More expensive (£60-200)
More complex installation (need to install shunt)
Need programming for battery capacity

My experience: Van #4. Victron BMV-712 (£185). Absolutely worth it. Transformed my power management.

Verdict: This is what you should buy if you’re serious about managing power.

Popular models:

Budget (£60-£90):

Renogy 500A Battery Monitor: £65
Basic features, adequate for most people

Mid-range (£120-£150):

Victron BMV-700: £125
Excellent, no Bluetooth (display only)

Premium (£180-£220):

Victron BMV-712: £185 (what I have)
Bluetooth to phone app
Multiple battery banks
Midpoint voltage monitoring
This is what I recommend

Professional (£250-£350):

Victron SmartShunt: £120 (shunt only, no display)
Victron BMV-712 Smart: £220 (all features)
Multiple device integration

Type 4: All-in-One Systems (Inverter/Charger with Built-in Monitoring)

What it is: Combined inverter/charger/monitor systems

Examples:

Victron MultiPlus: £800-£1,400
Includes inverter, charger, and monitoring

Pros:

Everything integrated
Professional grade
Single system

Cons:

Very expensive
Overkill for most vans
Complex installation

My experience: Never used (overkill for my needs).

Verdict: Only for very high-end builds or full-time professional conversions.

My Current System: Victron BMV-712 Detailed

Since this is what actually works in real life, let me break down exactly what I have and how it performs.

System:

Victron BMV-712 battery monitor: £185
500A/50mV shunt (included)
Temperature sensor (optional, I don’t use it)
Bluetooth built-in
VictronConnect app (free)

Installation location:

Shunt: In negative cable, within 30cm of battery
Display: Mounted on wall above seating area (visible from bed and seating)
Temperature sensor: Not installed (can’t be bothered)

Battery monitored:

Fogstar Drift 105Ah lithium
Programmed as: 105Ah capacity, lithium chemistry

What the display shows:

Main screen:

Voltage: 13.2V
Current: -8.2A (negative = discharging)
State of charge: 73% (77Ah remaining)
Time remaining: 9.4 hours

Secondary screens (cycle through with button):

Power: -98W
Consumed Ah: 28Ah
Temperature: — (no sensor fitted)

History data:

Deepest discharge: 62Ah
Average discharge: 31Ah
Charge cycles: 187
Full discharges: 0
Synchronizations: 187

What the Bluetooth app shows (same data, more detail):

On phone, I can see:

All current values (real-time)
Historical graph (voltage, current over time)
Trends (daily consumption for past weeks)
Settings (program capacity, parameters)
Alarms (set low voltage, low state of charge warnings)

Accuracy after 18 months:

I’ve tested accuracy multiple times:

Discharge battery from 100% to 20%
Count actual Ah consumed (should be 84Ah for 105Ah battery)
Monitor shows: 82-85Ah consumed
Accuracy: Within 2-3%

That’s good enough.

What I actually use it for:

1. Morning check:

Wake up, look at display
See state of charge (typically 65-75% after night)
Decide if I need to charge today

2. Real-time monitoring:

Turn on heater: See current jump to 12A
Laptop charging: See current increase by 6A
Everything turned off: Should see <1A (parasitic draw)

Helps identify power hogs and problems.

3. Planning:

Evening: Check state of charge
See remaining capacity
Decide: Can I run heater overnight? (yes if above 60%)
Or: Need to charge tomorrow? (yes if below 40%)

4. Troubleshooting:

Something drawing power when everything’s off?
Check current: If showing 3A with nothing on, something’s wrong
Hunt down the problem

I found my fridge was stuck “on” this way (current showed 4A when fridge should have been off). Thermostat had failed. Fixed it before it drained battery overnight.

5. Solar performance:

Check how much solar generated today
Yesterday: 42Ah
This week average: 38Ah
Helps me know if solar is working properly

Value provided:

1. Battery life extension:

I never discharge below 20% now (monitor warns me)
AGM batteries last 50% longer when not deep discharged
Lithium batteries last 30% longer
Value: £100-150 in battery life

2. Confidence:

I know exactly what my power situation is
No stress, no guessing
Value: Priceless

3. Efficiency:

I identified power-wasting devices (old LED lights drawing more than expected)
Optimized usage patterns
Value: 10-15% power saving = less charging needed

Payback: The £185 has already paid for itself in battery life extension alone. Everything else is bonus.

Installing a Battery Monitor: Complete Guide

For Victron BMV-712 (other shunt-based monitors similar)

Components Needed

Included with BMV-712:

Display unit
500A/50mV shunt
10m cable (display to shunt)
Fuse (for positive connection)

Not included (you need to buy):

Cable for positive voltage sense (1.5mm², 2m): £3
Battery cables for shunt (35mm² or 50mm²): £15-25
Terminals for cables: £8
Heat shrink: £5

Tools needed:

Cable cutters
Ratchet crimpers
Heat gun
Screwdriver
Drill (for mounting display)

Installation Steps

Step 1: Disconnect Battery (CRITICAL)

Safety first.

Turn off all systems
Disconnect negative terminal from battery
Wait 5 minutes

Step 2: Decide Shunt Location

Requirements:

In negative cable between battery and all loads
Within 30cm of battery
Accessible (you might need to check connections)
Dry location

Critical: ALL negative connections must go through shunt. This includes:

Loads (lights, fridge, etc.)
Chargers (solar, DC-DC, mains)
Inverter

If any negative bypasses the shunt, monitoring will be inaccurate.

My location: Right next to battery, on the wall. Negative from battery goes to shunt. Negative from loads connects to other side of shunt.

Step 3: Install Shunt

Shunt has two sides:

Battery side: Connects to battery negative
System side: Connects to all loads/chargers

Process:

Cut negative cable from battery:
- This is scary but necessary
- Cut it about 20cm from battery terminal
Install terminals on cut cable:
- Battery end: Crimp M8 ring terminal
- System end: Crimp M8 ring terminal
Connect to shunt:
- Battery cable to shunt “battery” side (marked “-“)
- System cable to shunt “system” side (marked “load”)
- Tight connection (shunt gets warm under load)
Label clearly:
- “Battery” and “Load” sides
- Future you will thank present you

Step 4: Run Cable to Display Location

The 10m cable (included) connects shunt to display.

Cable routing:

From shunt to display location (wherever you want display)
Can run with other cables
Needs protection where it might chafe

My route: Along wall, cable-tied to existing cables, to display location above seating. About 3m total.

Step 5: Install Display

Location choice:

Visible from main living area
Easy to read
Not in direct sunlight (can’t read display)

I mounted mine on wall above seating. Can see from bed, from seating, from kitchen.

Mounting:

Two screws (included)
Plastic snap-fit bezel
Simple

Step 6: Connect Positive Voltage Sense

Display needs positive voltage reference.

Cable from battery positive:

1.5mm² cable (adequate for voltage sensing)
FUSED at battery end (2A fuse, included with monitor)
To display unit

My cable: 2.5m run from battery to display.

Critical: This cable must be fused at battery. If it shorts, you need protection.

Step 7: Reconnect Battery

Order:

Connect display cables to shunt (included cable)
Connect positive sense to display
Connect battery negative to shunt battery side
Connect battery positive (with fuse for monitor)

Display should power on when battery connected.

Step 8: Programming

Display will show “—” initially. Needs programming.

Settings to configure:

1. Battery capacity:

Enter actual capacity (mine: 105Ah)
Critical for accurate state of charge

2. Charged voltage:

Voltage at which battery is considered 100% full
AGM: 14.4V
Lithium: 14.4V
Mine: 14.4V

3. Tail current:

Current below which charging is considered complete
Typically 4% of capacity
Mine: 4.2A (4% of 105Ah)

4. Peukert exponent:

Compensates for capacity loss at high discharge rates
AGM: 1.25
Lithium: 1.05
Mine: 1.05

5. Charge efficiency factor:

Accounts for charging losses
AGM: 85%
Lithium: 99%
Mine: 99%

Programming via Bluetooth app is easier than button interface. I used the app.

Step 9: Synchronization

Monitor needs to “synchronize” to know battery is at 100%.

Process:

Charge battery fully (14.4V+ for lithium)
Monitor detects full charge
Monitor resets state of charge to 100%
From then on, counts Ah consumed

First synchronization: Charge battery with DC-DC or solar until voltage reaches 14.4V and holds for 5 minutes. Monitor will synchronize automatically.

My installation:

Time: 3 hours (including programming and testing)

Challenges:

Cutting the negative cable felt wrong (but essential)
Cable routing through existing loom (a bit fiddly)
Programming settings (read manual twice to understand)

Result: Flawless. 18 months later, still working perfectly.

Using Your Monitor: Real-World Tips

Having a monitor is one thing. Using it effectively is another.

Daily Routine

Morning:

Check state of charge (typically 65-75% after night with heating)
Note overnight consumption (usually 15-20Ah with heating)
Decide if charging needed today

During day: 4. Monitor current draw when turning things on (is fridge actually cycling off?) 5. Check solar generation mid-day (should be charging if sunny)

Evening: 6. Check state of charge (plan for overnight) 7. Check total consumed today (typically 28-35Ah) 8. Decide on heating (can run if above 50%)

Weekly: 9. Check history (average consumption, trends) 10. Verify solar generation patterns

Takes maybe 2 minutes total per day. Worth it.

Reading the Data Correctly

State of charge:

Above 80%: Excellent, no worries
60-80%: Good, normal range
40-60%: Adequate, start planning to charge
20-40%: Low, charge soon (AGM shouldn’t go lower)
Below 20%: Critical, charge immediately

Current draw:

0-5A: Normal (just fridge and parasitic)
5-15A: Moderate (lights, laptop, water pump)
15-30A: High (heater on high, inverter load)
Above 30A: Very high (inverter with kettle, etc.)

Time remaining:

Above 10 hours: Comfortable
5-10 hours: Fine but monitor
2-5 hours: Plan accordingly
Below 2 hours: Charge soon or reduce load

Remember: Time remaining assumes current draw stays constant. If you turn heater off, time remaining increases.

Setting Alarms

Useful alarms to set:

Low voltage alarm:

AGM: 12.0V (about 30% full)
Lithium: 12.5V (about 10% full)
Warns before battery too low

Low state of charge alarm:

30% for AGM
20% for lithium
I have mine set to 25% (plays it safe)

High current alarm:

Set to 40A (would indicate problem)
Never triggered (max draw is 25A with everything on)

I use state of charge alarm primarily. Phone beeps when battery below 25%. Very useful.

Identifying Problems

Problem 1: Current draw higher than expected

Example: Everything off but showing 3A draw.

Diagnosis:

Turn things off one at a time
Watch current
Identify culprit

Found fridge was malfunctioning this way (drawing 4A constantly instead of cycling).

Problem 2: Battery not charging

Example: Solar panels in sun but current shows 0A charging.

Diagnosis:

Check solar controller
Check connections
Check panel cleanliness

Found dirty panels this way (output was 30% normal).

Problem 3: Capacity degrading

Example: Battery capacity seems lower than programmed.

Diagnosis:

Check history (deepest discharge should match capacity)
Run full discharge test
Battery might be aging

My 105Ah lithium after 18 months: Still showing 102-103Ah usable. No degradation.

Problem 4: State of charge drifting

Example: State of charge shows 45% but battery voltage is 13.2V (should be 70%+).

Diagnosis:

Monitor needs re-synchronization
Charge to 100% to reset

Happens if battery never reaches full charge (synchronization voltage). Easy fix.

Power Consumption Analysis: What I Actually Use

Having accurate monitoring let me see exactly where power goes.

Typical Day Breakdown (Summer, No Heating)

Device	Power (W)	Hours/Day	Ah/Day
LED Lights (4x)	15W	4h	5.0Ah
Fridge (cycling 50%)	45W	12h	22.5Ah
Water Pump	40W	0.25h	0.8Ah
Laptop Charging	65W	2h	10.8Ah
Phone Charging (2x)	15W	2h	2.5Ah
USB Devices	10W	4h	3.3Ah
Parasitic Draw	5W	24h	10.0Ah
Total			55Ah

Actual measured consumption: 58Ah (close to calculation)

Typical Day Breakdown (Winter, With Heating)

Device	Power (W)	Hours/Day	Ah/Day
LED Lights	15W	6h	7.5Ah
Fridge	45W	12h	22.5Ah
Water Pump	40W	0.25h	0.8Ah
Laptop Charging	65W	2h	10.8Ah
Phone Charging	15W	2h	2.5Ah
Diesel Heater	25W avg	10h	20.8Ah
USB Devices	10W	4h	3.3Ah
Parasitic Draw	5W	24h	10.0Ah
Total			78Ah

Actual measured consumption: 82Ah (heater uses slightly more on startup)

Surprises From Monitoring

1. Parasitic draw was higher than expected

Expected: 2A (lights on circuit boards, controllers) Actual: 5A (found old phone charger that drew 3A even with nothing plugged in)

Saving: 3A x 24h = 36Ah per day (nearly 50% of fridge consumption!)

2. LED lights varied widely

Cheap LEDs: 6W each Good LEDs: 4W each

Replaced 4 lights, saved 8W total = 32Wh per day = 2.7Ah

Small but adds up.

3. Laptop charging more efficient via inverter than 12V

12V car charger: 90W draw (inefficient conversion) 230V inverter + laptop charger: 75W draw (laptop charger is efficient)

Counter-intuitive but true. Saved 15W per charging session.

4. Fridge cycling was inefficient

Original fridge: Cycled 60% of time (too often) Reduced thermostat 2°C: Now cycles 40% of time

Saving: 9Ah per day (20% reduction)

Total savings from monitoring and optimization: About 15Ah per day (20% reduction)

This extended battery life from 2 days to 2.5 days between charges.

Integration With Other Systems

Battery monitor doesn’t exist in isolation. It integrates with other electrical components.

Solar Controller Integration

My Victron MPPT controller and Victron BMV-712 can share data via Bluetooth.

What this means:

MPPT can see battery state of charge
MPPT adjusts charging accordingly
BMV can see solar generation
Both visible in one app

Practical benefit:

On VictronConnect app, I can see:

Battery: 73% (77Ah remaining)
Solar: Charging at 48W (4A)
Time to full: 3.2 hours

All in one screen. Very useful.

DC-DC Charger Integration

My Victron Orion DC-DC also connects via Bluetooth.

What this means:

DC-DC sees battery state
Adjusts charging profile
BMV sees charging current
All data in app

Practical benefit:

When driving, app shows:

Battery: Charging at 18A
DC-DC: Bulk phase, 93% efficiency
Estimated time to 90%: 1.2 hours

Helps me know how long to drive for charging.

Multiple Battery Banks

Some monitors (including BMV-712) can monitor two battery banks.

Use cases:

Starter battery + leisure battery
Two separate leisure banks
Lithium primary + AGM backup

I only monitor leisure battery (starter is fine with vehicle alternator).

But if you’re paranoid, you can monitor both.

Cost-Benefit Analysis

Let me be honest about whether £185 for a battery monitor is worth it.

Costs:

Initial:

Victron BMV-712: £185
Installation materials: £15
Installation time: 3 hours
Total: £200 + 3 hours

Ongoing:

Maintenance: None
Calibration: Automatic
Total: £0/year

Benefits:

1. Battery life extension:

Without monitoring (my Vans #1-3):

AGM batteries: 2-3 years average
Lithium batteries: 6-8 years estimated (based on poor management)

With monitoring (Van #4):

AGM would last: 4-5 years (50% longer due to better management)
Lithium will last: 10-12 years (better cycling, no deep discharge)

Value for my 105Ah lithium (£449):

Without monitoring: £449 / 8 years = £56/year
With monitoring: £449 / 11 years = £41/year
Saving: £15/year in battery costs

Over 20 years (two battery replacements): £300 saved

2. Reduced charging costs:

Power optimization (from monitoring data):

Reduced consumption 15% (from 65Ah to 55Ah daily in summer)
Less frequent charging needed
Fewer campsite stays (solar covers more days)

Estimated saving:

3 fewer campsite nights per year @ £25 = £75/year

3. Reduced stress and decision-making:

Without monitoring:

Constant worry about battery state
Conservative (charge too often, waste time)
Or aggressive (run too low, damage battery)

With monitoring:

Know exact state
Make informed decisions
No stress

Value: Difficult to quantify but significant. £50/year equivalent?

Total annual benefit:

Battery longevity: £15/year
Reduced charging: £75/year
Stress reduction: £50/year
Total: £140/year

Payback: £200 / £140 = 1.4 years

After payback, it’s pure value.

I’m in year 2. Monitor has paid for itself. Years 3-20 are savings.

Verdict: Absolutely worth it if you use the van 50+ nights per year. Marginal if you use it 10-20 nights. Skip it if you use it 5 nights.

Budget Alternatives

Not everyone wants to spend £185 on a monitor.

Option 1: Basic Shunt Monitor (£60-£90)

Example: Renogy 500A Battery Monitor (£65)

What you get:

Shunt-based (accurate)
Amp-hour counting
State of charge
Basic display
No Bluetooth

What you miss:

Bluetooth monitoring (must look at display)
App features (history, trends)
Integration with other devices

Verdict: Adequate for budget builds. 80% of functionality for 35% of cost.

Option 2: Voltmeter + Current Meter (£15-£30)

What you get:

Voltage display
Current display (if you buy shunt separately, £12)
Basic info

What you miss:

No amp-hour counting (still guessing state of charge)
No history
No integration

Verdict: Better than nothing. Helps you see current draw (useful). But still doesn’t solve state of charge problem.

Option 3: DIY with Arduino/Raspberry Pi (£40-£80)

For the technically inclined:

Current sensor module (£15)
Voltage divider (£3)
Arduino or Pi (£25-£40)
Display (£8-£20)
Code (free, plenty of examples online)

Pros:

Customizable
Learn something
Cheaper than commercial

Cons:

Time investment (10-20 hours)
Programming required
Less reliable than commercial
No warranty

I considered this. Too much hassle for me. Would rather spend £185 and have it work.

My recommendation:

Tight budget: Renogy 500A monitor (£65) Normal budget: Victron BMV-712 (£185) Tech enthusiast: DIY project (£40-80 + 20 hours)

Common Mistakes With Battery Monitors

Mistake 1: Not Installing Shunt Correctly

What people do: Install shunt in positive cable (wrong)

Why wrong: Current flow must be measured in negative. Shunt in positive doesn’t work with monitor design.

Or: Some loads bypass shunt (separate negative return)

Result: Inaccurate monitoring (not counting all current)

Fix: ALL negatives must go through shunt.

Mistake 2: Wrong Capacity Programming

What people do: Program wrong capacity (e.g., 100Ah when battery is actually 95Ah)

Result: State of charge is inaccurate (thinks battery bigger than it is)

Fix: Program actual capacity. Check battery specs.

I initially programmed 105Ah. After testing, realized actual capacity was 102Ah. Reprogrammed. More accurate.

Mistake 3: No Synchronization

What people do: Never charge battery to full (so monitor never synchronizes)

Result: State of charge drifts over time (accumulating small errors)

Fix: Charge to 100% at least monthly (allows synchronization)

Mistake 4: Ignoring the Data

What people do: Install monitor, never look at it

Result: Wasted money (monitor sitting there unused)

Fix: Actually use the data to manage power

Seems obvious but I’ve met people with monitors who admit they rarely look at them.

Mistake 5: Over-Reliance on Time Remaining

What people do: Trust “time remaining” estimate absolutely

Reality: Time remaining assumes current draw stays constant. If you turn heater on, time remaining drops. If you turn everything off, it increases.

Fix: Use time remaining as guide, not gospel.

My Final Recommendations

After running three vans without monitoring and one with proper monitoring:

For regular van users (50+ nights/year):

Buy Victron BMV-712 (£185).

Why:

Accurate monitoring (shunt-based)
Bluetooth to phone
Historical data
Integration with Victron components
Reliable (18 months, zero issues)

Total cost: £200 including installation materials

For occasional users (20-40 nights/year):

Buy Renogy 500A monitor (£65) or similar budget shunt monitor.

Why:

Adequate monitoring for occasional use
Saves £120 vs Victron
Still accurate (shunt-based)

For weekend warriors (10-20 nights/year):

Consider skipping battery monitoring entirely.

Why:

£65-£185 is significant cost for limited use
Basic voltage monitoring might be adequate
Simpler system (less to go wrong)

Alternative: Just keep charging conservative. Charge when voltage drops to 12.3V. Not optimal but works.

For full-timers or off-grid enthusiasts:

Buy Victron BMV-712 without question.

Why:

Essential for managing power properly
Battery life extension alone pays for it
Peace of mind is invaluable
Integration with solar/charging is useful

My Current System Summary

Monitoring:

Victron BMV-712: £185
Installed 18 months ago
Zero issues

Battery:

Fogstar Drift 105Ah lithium: £449
Monitored constantly
187 charge cycles so far
Still at 98% capacity (102Ah usable)

Charging:

200W solar + Victron MPPT: £265
Victron DC-DC 18A: £157
All visible in VictronConnect app

Integration:

All three Victron devices share data
One app shows everything:
- Battery: 73% (77Ah remaining)
- Solar: 42W charging
- DC-DC: 18A charging (when driving)
- Today consumed: 28Ah
- Today generated: 36Ah (net positive)

This is the power management system I wish I’d built in Van #1.

Total cost: £185 (monitor only) or £607 (entire system)

Value provided: Immeasurable. Complete confidence in power situation. No stress. No guessing. No damaged batteries.

Final Thoughts

I spent three years and two batteries (£330) learning that guessing battery state from voltage doesn’t work.

A voltmeter tells you almost nothing useful. Voltage-based “smart” monitors are barely better. Both are guessing, and guessing wrong leads to damaged batteries, stress, and poor decisions.

Proper monitoring – shunt-based amp-hour counting – transforms power management. You finally know what’s actually happening. Make informed decisions. Optimize usage. Extend battery life.

Is £185 expensive? Initially, yes. But it’s paid for itself in battery longevity and reduced charging costs. Everything beyond payback (18+ years) is pure value.

If I built van #5 tomorrow, the Victron BMV-712 would be installed on day one, before I installed lights or fridge. That’s how fundamental it is.

Stop guessing. Start monitoring properly. Your battery will thank you.