I have a pair of 6v batteries (Trojan T-105) with a nominal 225ah capacity. They are currently a little over 7 years old, and doing just fine as far as I can tell... Thing is, how do I know when they begin to lose capacity?
I have a Link-10 battery monitor (amp counter) that also shows me the % of battery charge left, but it's pretty dumb, right? It has no way of knowing if my cells are losing capacity, does it? I generally limit my discharge to 110ah (50% DOD) before recharging, and that's only a few times a year. Usual use is much shallower than that. When at home, it's kept full time on a charger, a PD-9160 with the 'charge wizard' that floats the batteries at 13.2v.
I check water levels and top them up once per year, and they've never dropped below the plates. Twice in their 7 year life I accidentally flatlined them while in storage, both times they seem to have recovered without immediate damage.
So... I know I'm at the tail end of a normal service life for these batteries, and I have no problem replacing them with new ones if needed, but so far they seem to be going strong! But... I don't want to find out in the middle of a winter storm in December that I only have 80 ah available instead of the 110 that I think I have...
Is there any way to determine the capacity of flooded lead cells short of cycling them down to dead and measuring how many amp-hours it takes?
you can go to a battery store or maybe a place that has a load tester for batteries. placing the batteries under load is one way you can tell. using a hydrometer will tell you if the batteries are OK also.
The way I notice things changing is when recharging from 50% with the usual charging rate, is if the time at constant amps is shorter that means capacity is less.
You have to adjust your notion of what is the proper capacity for the temperature, but if the time in bulk is still "too short" then you know.
The other way is with the monitor and the accumulated AH since your last recharge to what you believe is full charge. If the "morning voltage" is 12.1 and hydrometer says 1.175 (T-105 at 50%) but not enough AH have been used, you know you either were not really full to begin with, or that your capacity is reduced.
To confirm, if I am suspicious, I do in fact run 10 hr load tests at home at the 20 hr rate draw (using an inverter and lamps at the right wattage) and note the time to reach 50% SOC -measured by SG and by voltage on bounce back after load is removed. If it only took 8 hours to get to 50% your capacity is reduced.
How much reduction is acceptable depends on your situation. A steady decline is ok, but you don't want a rapid fall -off like at the end-of- life while camping. So you have to get new batteries "early" unless you are camping near a store that sells batteries.
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I Too use an inexpensive carbon pile load tester (The one with two meters on it from Harbor Freight).
But to be honest if you really want to know the capacity of the batteries you need a good battery analizer. This will first charge them, then rest them a bit, then load them down and time the discharge, once they reach a preset point of discharge it can calculate the battery's amp hour capacity, Re-charge and try again at a different load rate, and re-figure it, I would guess that with a computer doing the math no more than 2 or 3 data points and you will have a true "20 hour curve" figure.
These are kind of expensive though.. Thus not practical.
But a battery store might be able to do a decent job, IF you can not find a Trojan Distributor Check with other battery stores.
I know some of the battery testers do not do the full range of tests but alas, Can not tell you which ones.
The load tester measures internal resistance more than capacity, but as the one goes up (Resistance) the other usually goes down (Capacity).
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It's not clear to me how a load tester tells me much. I mean, I know they are in good shape, pulling a load shouldn't be a problem at all. I gather it measures internal resistance, then somehow estimates capacity from this? Is that correct? Is this accurate?
I own a little battery workstation (Cellpro PowerLab 6) that I've done a lot of battery cycling with, but only with LifePo4 cells, not lead. I could do a cycle test with that I guess, though I'm not sure what voltage to use for the 'discharged' value. Li cells have such a flat curve it's easy to tell when they fall off the knee at the end of cycle (flat line that suddenly plummets, cycle to 2.8v and you've reached pretty much fully discharged with LifePo4).
For simplicity's sake, I am thinking I could just turn on a few lights until I'm pulling somewhere around 10a, then leave it on until I reach 110ah out on the Link-10. Let the cells rest a few hours, and measure the resting voltage to see how close to 50% DOD I am. Do you think that would be useful info? I know state of charge and voltage are very directly related with lead batteries.
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Voltage and specific gravity can tell you state of charge, but not capacity. The battery could be 600 AH or 50 AH and still have the same SOC and could all have the same voltage/SG numbers at 50% depth of discharge (DOD). Your proposed test of drawing a known number of AH (you should pull at the 20 hour capacity rate) then measuring SOC is a decent way to get a capacity estimate. You'll find it's not easy, however, as you have to get it fully charged (best to keep charging until SG no longer changes and I'd equalize after the first charge, then keep charging to get the last little bit in there) and measuring SOC tends to be tricky. I keep getting different numbers with different methods.