Motorhome batteries – WorldofWills

There is a lot of “discussion” in forums about the type of battery that should be fitted in a motorhome, and why some types are deemed to be unsuitable. In order to help me decide on which battery to buy to replace our existing battery I have pulled together the following information.

Summary of battery types

Wet lead acid: traditional old fashioned wet cell technology. Charging the battery heats the electrolyte and causes gassing.These batteries must be maintained by topping up with distilled water. They must also be vented and should NOT be mounted inside a motorhome. As the electrolyte is a liquid, wet cells are more susceptible to vibration. Some are sealed so shouldn’t require topping up. Electrolyte is liquid so can spill in a crash.

EFB: Enhanced flooded battery: A step up from the wet cells, still a wet cell battery but does not need topping up. Supposedly sealed but still should be vented. Electrolyte is liquid so can spill in a crash and they will be susceptible to vibration. Charge regime as per wet lead acid battery.

Absorbed Glass Matt (AGM): this is the most contentious battery type. It was introduced as a completely sealed, high power, deep discharge, rapid charging, high performance battery. They are less susceptible to vibration as their electrolyte is encapsulated in the glass matt. AGM batteries are fully sealed and do not need venting. They can be mounted inside the motorhome.

Gel: Gel batteries are another deep discharge, rapid charging battery type that are completely sealed. Due to their construction, gel batteries can be mounted at any angle or orientation and are suitable for mounting inside a motorhome.

Gel batteries perform better than AGM when deep discharged, meaning that they will provide a greater number of recharge cycles, they perform better at higher temperatures than AGM and are less sensitive to thermal runaway.

Gel batteries recharge much faster than flooded-cell batteries, provided that a regulated charge voltage is temperature compensated, but cannot be charged as quickly as AGM.

Gel batteries are extremely sensitive to charge voltages above 14.1-volts DC at 68 degrees.

There are many posts in the forums from people whose batteries have failed after 1 or 2 years and many blame the AGM technology.

My current battery is a Banner AGM. It is 5 years old and still retains its charge. My “usage pattern” is one of low rate of discharge – we have LED lighting and rarely use the TV; we also keep it on trickle charge when the ‘van is parked at home. We have not gone off-grid for more than 3 days at a time, the last time being in the New Forest in October where we needed lighting, water pump, satellite (radio) and heating. The battery discharged to 12.3volts by the end of the 3 days and was showing as fully charged by the time we arrived home, less than an hour’s drive away.

Much of the “noise” around the suitability of AGM batteries seems to be that they are unsuitable if you cannot charge the battery at 14.7V. The following graphic is from the Victron site and clearly shows that the normal charging voltage for an AGM battery should be between 14.2 and 14.6 volts. The ‘van’s alternator voltage is 14.4 and the EBL is capable of 14.4V which fall in the normal service charge range for an AGM battery.

From the Victron table above it looks like 14.6-14.9V is needed to perform a fast recharge of the battery as opposed to a ‘normal’ recharge.

Banner specify a maximum charging voltage for AGM at 14.8V.

An AGM is capable of a fast recharge which would mean supplying 14.6-14.9 volts.

An AGM has very low internal resistance allowing it to be charged very quickly. Low internal resistance combined with a low state of charge and applying >14.4V will lead to a high current charging so will necessitate using a charger capable of providing around 25% of the battery rating to adequately charge. For a 100Ah battery, the charger should be rated at least 25A.

Low internal resistance also leads to better high current discharge rates than wet cell batteries hence exhibit a greater number of charge-discharge cycles.

AGM batteries require precise, voltage-regulated, temperature-compensated charging routines. AGM batteries can accept a very high charge rate, a charge rate unlikely to be provided by most van battery chargers. Chargers should be connected to a battery mounted temperature sensor to prevent thermal runaway.

Gel batteries recharge much faster than flooded-cell batteries, provided that a regulated charge voltage is temperature compensated.

Gel batteries are extremely sensitive to charge voltages above 14.1-volts DC at 68C.

Notes:

Our on-board Schaudt EBL 29 has battery charge settings for “wet” and “gel”, but does not have a setting for AGM so cannot provide the higher 14.6-14.9V required to rapidly recharge an AGM.
The Schaudt EBL 29 contains a Schaudt LAS 1218 charger and can provide up to 18A charging current.
The Schaudt EBL charges both the habitation and cab batteries.
The Fiat cab alternator supplies a charging voltage of approx 14.4V

Thoughts:

Wet cells, even the EFB (enhanced flooded battery) are really only capable of a maximum of around 200 recharge cycles. They are not capable of rapid charge and discharge, they will also need to be vented.
EFB cells are really only suitable for low power requirements where the rate of discharge is low.
Adding a second habitation battery may require a second charger to be fitted although the EBL could charge 2 95Ah batteries, but this would be an edge condition and is outside the spec of the EBL29. As gel batteries charge at a slower rate than AGM, the EBL could support 2*80Ah gel batteries.
A second charger could be connected directly to the batteries or connected as an auxiliary charger through the EBL. The EBL auxiliary charger is fused at 20A charge current. A suitable charger is the “blind” Apuljack Engineering AE276Plus or one of the Victron Blue Smart chargers, in particular the 15A charger.
A Gel battery will typically provide 600 recharge cycles
An AGM battery will typically provide 400 recharge cycles
An AGM can be charged at a faster rate than Gel or wet.
A wet or EFB battery will be unlikely to provide up to 200 cycles

Single replacement battery costs

	Wet/EFB (100Ah)	AGM (95Ah)	Gel (80Ah)
Cost	120	170	180
Additional charger		–
Total	120	170	180

Dual replacement battery costs

	Wet (100Ah)	AGM (95Ah)	Gel (80Ah)
Cost	240	340	360
Additional charger	140	140	0
Sundries (cable, sockets)	40	40	40
Total	420	520	400

Conclusion

EFB batteries are not suitable for any kind of wild camping as they exhibit too few charge/recharge cycles.

Both AGM and Gel batteries require a well-regulated charging system. Without a voltage regulated, three-stage battery charger with temperature compensation then you should not expect to achieve the battery’s rated life cycle.

With a correctly rated charger capable of 3+ stage charging and temperature compensation AGM should provide a long lasting, high performance battery power

In addition, the AGM and gel’s self-discharge rate is extremely low. Flooded-cell batteries self-discharge 1 percent per day or 30 percent per month (at 68 degrees), while VRLA batteries self-discharge generally less than 2 percent per month.

Habitation batteries will not recharge fully from the van alternator and should be regularly charged using a well regulated 3+ stage charger.

We have to consider the motorhome 12V power supply system in its entirety and not just look at individual components. Focussing on the battery without addressing its usage or charging requirements will result in less than optimal power delivery and increased costs.