are all my battery Ah usable?

[gerry mcg]

Hopefully a simple question...
If I've got a 100Ah battery, Are all these Ah 'available'? I.e Within the 50% usable capacity of the battery before the voltage would drop below the point where you risk damaging the battery?
Or is there usable capacity more like 50Ah?

 

[BwB]

50Ah

 

[icantremember]

Yes, you would have 50% useable power before the battery will start to deteriorate if not recharged.

 

[eddie]

Depends on the battery, Lithium iron 90% - 95%

A tad more expensive though

 

[gerry mcg]

many thanks!

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[Wildbill]

Make sure you use a recovery charger as running your battery low will result in sulfated battery plates.
Recharge at higher voltage to remove the sulfate
Or desolve it back to usabill active electrolyte
Bill

 

[gerry mcg]

I'm just talking theoretically at the moment.

 

[two]

It’s best to assume no more than 50% of the claimed capacity (unless it’s Lithium). It will be less still in extreme cold and older batteries less so again. And, if it was not a quality battery, the rated capacity may well have been optimistic.

 

[aandncaravan]

Yes, but no?.
There is no real limit with a Lead Acid battery, you can take as much as you like, but the penalties will be greater the deeper you discharge.
Exide, Yuasa, Varta, etc suggest the ideal discharge for optimum life is around 30%. You can discharge lower but the cycle count can drop dramatically.

50% is suggested as a reasonable compromise discharge level, but the useful life time of the battery can be significantly shorter than at 30% discharge.
While most conventional battery manufacturers suggest 50% should the max discharge on a regular basis, the depth of discharge is your choice, versus lost life.

On lower quality batteries permenant damage can occur below 70% discharge. That damage may prevent the battery from recharging fully or even at all, 60% is what we advise as the absolutely lowest discharge.

If you look at the attached chart, published by Exide, at the upper line for the Exide Gel you will see that you can discharge a battery as low as you like. But as you get closer to 98% discharged you will only get only a handful of Charge/Recharge cycles.
If you discharge to 25%, you will get around 1,200 cycles.
Discharge to 50%, you will get around 840 cycles, etc.

This behaviour applies to all Lead Acid batteries, even the amaing Victron Energy Extra Long Life Deep Cycle Gel battery which gives nearly 5,000 cycles at 25% discharge, but just 1,500 at 80%. The Victron is a specialist Deep Discharge battery, but you can see the penality of over two thirds lost cycle life if you do Deep Discharge.

The second chart (published by Yuasa) gives you voltages that relate to a percentage discharge, but you need to know your battery type as different batteries have different resting voltages.
For example Gel, AGM and Varta LFD have around a 13v state, but a conventional battery like a Banner will have 12.6v as it's max.
So a VRLA (Gel/AGM) and Varta LFD's/Bosch L5's should ideally not drop below 12.5v, or 50% discharge.

However, remember that these are actual battery voltages, the display in the Motorhome, may read up to half a volt out so verify the accuracy of your display by borrowing a quality, accurate meter. Not one from Maplins!!!

And finally, whenever you do run any battery down low, think of the charging systems that have to charge it back up. Some won't cope with a very low battery (or even worse, two batteries) so just Pop.
While a battery may take a 60% discharge level, if you are at the limit of the charger capability with a bigger battery bank, then the chargers limit should be what you go by rather than what the battery will safely handle.

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[KeithMak]

and going off on a small tangent, the quoted capacity Amp.hrs is only at a specific temperature and discharge rate.

Batteries lose there capacity as they age, so although it states a certain number of Amp.hrs it is very rarely if at all you have that number.

I doubt in the motor-home world that anyone (there's bound to be one at least before anyone jumps in ) does a power audit and then builds the battery bank and charging system to cope.

Formula:
maximum use, (amp,hrs) doubled plus ten percent and that will give a minimum battery bank size.
Discharging to 50% SOC with a slight safety margin.

To only discharge to 75% SOC the maximum used would need to be quadrupled in the formula.

and of course whatever you use has to be put back plus a little more because of the losses when charging batteries.

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