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Topic: Real House Battery Capacity (Read 677 times) previous topic - next topic

Real House Battery Capacity

My coach came with 2 8G8D house batteries and that is what I have now.  These batteries are rated for 225 Ah each 450 Ah total.  How does the manufacturer come up with this rating?  My guess is that the battery when new and under ideal conditions is discharged 80% at the C20 rate and it produced 225 Ah.  Since we should never discharge below 50% for long life that would give a usable capacity of 140 Ah.  I arrived at this number by: 225 divided by 8 times 5 = 140.625.  Can any of the electrical experts confirm this?  My math and other reasoning may be faulty.

I know that keeping lead acid batteries fully charged promotes long life, which I do, but in most cases that is impossible, and somewhat defeats the purpose of having a battery in the first place.

Re: Real House Battery Capacity

Reply #1 –
The old heart freedom 25 manual mentioned what you are saying about the useable capacity to have efficient  gen run time.

The manual said from 50 to 85%. 

Visione rv recycling had a 97 U320 last I looked,  it had 3 8G8d's in the same compartment. It added a top rack.

So you would have 35% of 675 amp hours. 

For increased capacity and 20% faster recharging time in bulk a set of lifeline Agm's could be installed

35% of 500 amp hours.

Disadvantage?  The batteries can need equalizing if not charged at the 1/5th or 1/20th C and not brought up to full 100% at least weekly.

Requires excellent charging habits.

The mk gels do not need as much perfect charging habits

Re: Real House Battery Capacity

Reply #2 –
Quote from: Jerry Whiteaker –
My coach came with 2 8G8D house batteries and that is what I have now.  These batteries are rated for 225 Ah each 450 Ah total.  How does the manufacturer come up with this rating?

Lead acid battery numbers are soft because total battery capacity is determined by the rate of discharge.  The faster you discharge a lead acid battery the less total capacity you'll find.

BU-503: How to Calculate Battery Runtime – Battery University

If you want to explore the math behind battery discharge rates and capacities, here you go:

Peukert's law - Wikipedia

As far as battery damage goes, there are two types of damage.  One is the shedding of the lead sponge by its physical size change between full charged and fully discharged.  Think of the plate separators moving from one side to the other, from the positive plate to the negative one.  The other kind of battery damage is plate sulphation, but this takes time to occur to if you re-charge your discharged batteries immediately, no harm no foul.
As far as rate of charge is concerned, my opinion and practice is to provide all of the current the batteries can absorb at a regulated 14.4 VDC and watch the current flow taper off from whoopee to boring.  After four hours 80% state of charge will be achieved and you can turn your generator off. That last 20% needs to be done at 13.6 VDC and takes a long time, so it waits for shore power.
When dry camping, when my resting battery voltage measures 12.0 VDC I assume 50% SOC and begin the charging process.  Since the final chemical reaction takes the most time, rest periods during the charge cycle, I believe, help maximize the use of the generator's electricity.  When dry camping, one hour of generator for breakfast, one for lunch and after a voltage measurement one to two hours of generator run time for dinner.


Re: Real House Battery Capacity

Reply #3 –
Good info.  Since mid 1997 the oem installed heart freedoms have had the ability to add the necessary battery temp controlled charging.

All battery makers require this in their manuals.

14.4 in bulk is probably not correct at 100 degrees at the batteries. Or 13.6 float.

Your charger or batteries normally have correction charts showing you what manual adjustments should be made to charge the batteries correctly for their temps.

Just depends on how long you want your batteries to keep their capacities like they had after the first 15 cycles of break in.

Re: Real House Battery Capacity

Reply #4 –
Quote from: Old Toolmaker –
d you can turn your generator off. That last 20% needs to be done at 13.6 VDC and takes a long time, so it waits for shore power.
When dry camping, when my resting battery voltage measures 12.0 VDC I assume 50% SOC and begin the charging process.  Since the final chemical reaction takes the most time, rest periods during the charge cycle, I believe, help maximize the use of the generator's electricity.  When dry camping, one hour of generator for breakfast, one for lunch and after a voltage measurement one to two hours of generator run time for dinner.
With solar, no generator is needed and the solar controllers are smart so do the float voltages nicely. Plus, you can adjust the float to what ever voltage your manufacturer specifies.

Recycle those old generators.  :D

Pierce