Protecting Leisure Batteries and Smartifying Campervans
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This post is slightly different from the usual, as it's all to do with my campervan. As some of you might know, I enjoy camping and have converted a Renault Trafic into my very own campervan, equipped with all the mod-cons one might expect when camping in luxury.
One of the downsides to this luxury is that it all uses a lot of power... the lights draw about 10W all in, the inverter sips around 30W just at idle, and don't even get me started talking about the diesel heater. All this power has to come from somewhere, and for me, it's a leisure battery.
I purchased my leisure battery in November last year, after the previous one I had kept dying on me. I was just about to set off for a weekend in Bath where it was down to -2°C overnight, and really didn't fancy being without my diesel heater. When I popped into Halfords and handed them the tidy sum of around £230, I justified the replacement because I naïvely thought the old one had only died because it was old, and not because I didn't know how to look after them properly.
Battery Technologies
When shopping for leisure batteries, there are three main routes you can take:
- a regular car starter battery: good for putting out high currents but should be kept topped up, not recommended for use as a leisure battery
- AGM or EFB leisure batteries: similar technology to a car battery, but using some sort of material to hold the battery acid/electrolyte to allow you to discharge the battery more
- lithium-ion: the latest and greatest in battery technology but also the costliest and possibly most dangerous.
Regular Car Battery
You shouldn't really discharge a normal car battery much below its maximum for long periods of time. This is fine when cranking the engine, because as soon as it starts, the alternator gets to work charging the battery back up to full. Car batteries are good in harsh conditions and for where high currents are needed for short periods of time, but aren't really suited to slow, deep cycles.
AGM/EFB Battery
AGM (absorbent glass mat batteries), and their slightly cheaper but worse counterparts, EFBs (enhanced flooded batteries), are historically the best choice for leisure batteries. Compared to a standard car battery, they are more tolerant to a deeper depth of discharge, and therefore have a higher effective capacity than a regular car battery.
Manufacturers and advice on the internet all advocate for approximately a 50% depth of discharge, compared to their conventional lead-acid counterparts, meaning that someone with an AGM battery of say 100Ah would be able to use about 50Ah x 12V = 600Wh = 0.6kWh worth of charge before doing damage to their battery. Additionally, they are rated for a higher number of cycles and so can be charged and discharged more than a regular car battery.
Compared to lithium-ion, which is the most expensive and latest technology with the highest energy density of the lot, the AGM type of battery is a good budget-conscious choice for a campervan conversion.
Lithium-ion
Finally worth a mention is the state of the art in battery technology. Lithium-ion phosphate, or LiFePO4 is the current best choice in terms of energy density and longevity for a battery. Lithium-ion batteries are a fairly recent invention, and innovation has been spurred on by the advent of small handheld devices and now more recently electric cars. Lithium-ion batteries are the best in terms of energy density and allow a near 100% depth of discharge without doing immediate long term damage to the cells.
These types of batteries do however come with several disadvantages. The primary of these is the cost of the battery, using precious metals such as lithium over the more traditional lead, and the maturity of the technology compared to a traditional battery. They are also much pickier about how they are charged, and won't accept the output of an alternator from a van. Lithium batteries are also much more unstable and are a much greater fire hazard and require a smaller temperature range in which they can be charged and utilised.
Sizing Battery Requirements
When buying a battery, many sites give you calculators for how to size a battery for your requirements. Personally, I just put the biggest one I could into my van, which worked out as a 95Ah AGM battery, setting me back £230 last year–not a small investment at all, and something worth protecting.
My usage typically consists of being parked up for a few days without moving the van, using the inverter to power some string lights and my laptop, general lights in the van, and over the winter months, my diesel heater.
The biggest draw on the van is definitely the inverter and the diesel heater loads, with the diesel heater essentially running a glow plug for a couple of minutes to warm the chamber up, and the inverter typically using much less power but for a longer period of time.
So far, I have found that the battery I selected has been inadequate, and I am looking to replace it once the capacity is completely diminished. I regularly run past the 50% depth of discharge on the battery, which causes damage, and means that next time I use the battery there is slightly less capacity 
. I reckon a battery with about twice the useable capacity would be good for me, as would solar in the long run.
Measuring Discharge
Different technologies have different voltage levels and different curves for how they measure the discharge. AGM varies between 12.8V nominal a 100% charge, and 12V at 50% depth of discharge. Conventional lead-acid has a slightly different range, and lithium-ion is reasonably steady until fully discharged.
To measure discharge accurately, the use of a shunt or coulomb counter is recommended, using a series resistor with a known very low resistance to measure voltage drop, and therefore current passing a particular point in a circuit.
The main products I see that do this are the Victron BMS systems, which include a shunt and a small microcontroller, and can be programmed to detect the current state of charge of a battery system. These run about £110-130 and are much more useful with a lithium system, as opposed to an AGM system where a rough state of charge can be seen from the voltage with no load.
In the future, I'd like to look into installing a shunt and associated battery management system, but with my current usage and battery type, it's a slightly frivolous purchase.
Protection Systems
The main way I think most people damage their leisure batteries is to allow them to discharge more than they should. I know my Dad lets his get down to 9V on some evenings and now the battery barely holds a charge. The lowest I've seen mine is about 11.4V, but this is also far too low for a battery to stay healthy.
I decided that before I replace my current battery, I needed to fix what caused it to die in the first place, as it's lasted less than a year which when you think about it in terms of camping nights away is quite costly. This year, I reckon I've done maybe 30–40 nights of camping, which means the battery has effectively cost me £5–7 extra per night of use.
BatteryProtect
The system I landed on for ease of use is the Victron Smart BatteryProtect 12/24-Volt 100A, which makes use of a transistor as the electronic switch as opposed to a relay to allow current to flow. This switch automatically sounds an alarm when it's low on battery, before it switches off the system.
I can program it to cutoff at a predefined voltage which I can set to just above the 50% depth of discharge on my next battery. This goes in series with the main live to the fusebox to the rest of the van, but unlike a relay, the BatteryProtect system can only take one way current (i.e., all downstream devices must be a load on the battery and cannot send current back the other way).
I put this in my electrical cupboard, making sure to wire the inverter directly to the battery as it is a capacitive load. My inverter, being a cheap one, doesn't have a remote cutoff switch, so I had to do some minor disassembly and reverse engineering to make it work.
I removed the internal 12V that goes to the switch onboard, instead sourcing that externally. This means that I can kill the 12V to the inverter without disconnecting the main terminal posts, effectively creating my own remote switch. I then wired this in the van to an additional convenience switch, meaning the inverter switch in the cupboard can stay on the whole time and an easier, better placed switch outside the cupboard allows me to turn the inverter on and off.
Conclusion
Adding battery protection to a van's electrics is an easy enough job, and provides some additional peace of mind for the health of a battery, meaning that accidental or intentional over discharging is not possible. For the sake of around £60 all in, you can save future issues with over discharging and damaging your leisure battery.