Determining AH rating of aging batteries

Fisherguy wrote:

BFL13 wrote:
Run the ten hour test at the 20AH rate to see how long it takes to get to 50% (50% measured by SG and confirmed by voltage after waiting an hour after removing the load)

240AH 20 hr rate is 12 amps ( four 3 amp lamps or whatever) Use the Trimetric AH counter for more confirmation.

Say it takes 9 hrs to get down to Trojan's 50% SG of 1.172 and after an hour of bounce back you are nearing 12.1v so you are at about 50%

9 out of 10 is 90% or 216AH of 240 (Trimetric AH count for comparison cross-check)

If you use lamps then they dim as voltage drops and your 12 amps gets less so turn on more lamps as test goes on. If you use an inverter and some load to get 12a then you get inverter creep up in amps as voltage drops so in that case start with 11 and end with 13 to get 12 average.

If you are lower in capacity then try a serious session of overcharging at 15.5v to see if you can get the SG up if some of the sulfation is not yet permanent.

Without reading this post first, I ran a 12 amp load for 10 hours, the voltage was down to 11.9 on my trimetric, but after reading your post I shoulda let them 'come up' for an hour after my test?
Guess I gotta run it again..?

Not clear what you did. Sorry not enough internet access just now to read Mex's limk. Anyway my method has you starting with the batts as full as possible such as after the EQ run with the VEC1093. Now run them down at the 20hr rate.

You don't just let it run for 10 hrs. The whole idea is to see how long it really takes to get to 50%. You have to stop at 50% best as you can tell that. SG is the best way at that 1.172 for Trojans but also the loaded voltage will be around 11.6 when near 50% so you know be checking SG then.

When you are at 50% and load is removed voltage will bounce back from loaded 11.6ish to 12.1 if you are on target. After half an hour voltage rise gets slow so you can get a feel if you will overshoot 12.1 with more time. Say it comes out about right.

Now you have the time it took, say 9 hrs so your batts have 90% and the Trimetric AH reading for how much was drained as a cross-check for 50% gone so double that is your capacity.

You can confirm going the other way with the Tri AH to refill from 50%, but this can be off where they use 4% for heat loss and heat loss might be some other number, so the draw down test is better.

While camping you can get an idea from your 50-90 and the Trimetric AH that took too. You get to 50% the same way with SG and 12.1, but at various draw rates of course, then recharge till batteries are only accepting 5 amps per 110AH at about 14.5v. this is approx. 90%. So for 240AH you stop the charge when the 1093 gets down to 240/110 x 5 = 11 amps.

You have the AH that 40% took, say 90AH (The Tri has already allowed for 4% heat loss, so go with that) so 100% is 100/40 x90 = 225 and 225/240 = 94% in that case. Gives you a rough idea especially if it comes out the same a few times. (except successive 50-90s when your capacity will be less each time until you can do a 100% recharge to recover)