Calculating a mystery batteries Amp/hour capacity

[solarengineer]

I have previously posted picks of a battery bank I have, It's 6 2 volt cells

I have no idea of their capacity.. and there is not one sticker or marking anywhere on them.

I have built an amp/hour totalizer that keeps track of the amp-seconds going into the battery and converts and displays it in amp/hours.

What I'm trying to figure out is lets say I charge the mystery battery until it reaches

14.5V for a full hour or so. Whatever my amp/hour meter says went into the battery should roughly be its amp/hr capacity minus a guess on what percentage of the power going in went to losses.

so just pulling figures from a hat lets say:

200 amp hours went in to fully charge the battery to 14.5V including bulk phase/absorption phase.

now say 8% of the energy was wasted in the battery due to losses.

200. x .08 = 16 amp hours lost from the total leaving a battery capacity of
     
184. amp hours.
     
     

am i on the right track or heading for a crash? I would like to be able to program my controller to be able to adapt to battery age ect and be able to tell the actual measured amp hour capacity of the attached bank. If I can nail that down then I can apply Peukerts equation to the load current to determine approx run times at various current draws.

Thanks for any advice.

Jamie

[Flux]

Amp hour capacity is based entirely on discharge.

Fully charge it, with that type battery until it is gassing freely (over 15v) and the sg does not rise further after another hour of charge.

Then discharge it at a sensible rate ( say 10 to 20A and find the hours to reach about 10.5v.

That will give you a base to work on. From that you can repeat the test with a load to give you a C20 rate ( or whatever rate you want to rate it at).

Charge Ah will only give you a figure for Ah charge efficiency and does not affect the rated capacity.

Flux

[solarengineer]

Thanks Flux, would there be any other way of doing so that it doesn't have to discharge so deep? I'm worried about further hurting the batteries by such a deep discharge.

Is there some kind of general relationship like:

Battery = FULLY CHARGED no change in SG after being 15V for an hour or so.

Add a load for about an hour then take the battery voltage and apply a calculation to it to get approx amp hour capacity. Then repeat over and over to get an average.

The reason I need to do it similar to this is I would like to be able to use this controller on all kinds of different capacity lead acid batteries.

I want the controller to figure out the amp hour capacity of the battery with a calculation that happens over and over as the lights and various loads in the house cause a battery discharge. This would always be a varying load.

I'm trying to make a gas gauge style charge controller/meter similar to the emeter.

The meter over time could also show diminished battery capacity.

I know everyone says gas gauge style meters dont work great on batteries but still I would like to try.

Thanks

JAmie

[Flux]

I don't think you can do this without a full discharge, I don't see any way of determining other than full charge or full discharge. Don't worry about one complete cycle of discharge on a traction battery, it has been doing just that for all its life if used as intended. The issue with other batteries such as the marine ones may be more of a worry but even then if you want its true capacity I see no other way.

You can use an Ah meter as an energy meter but to know where you are you need to determine the true capacity. Once you have that you will need to take Peukert effect into account if you use widely varying load currents. If your load is reasonably constant you may just meed to establish Ah charge efficiency, which you can establish from the full charge point.

Get yourself to the Victron energy site and download the manual for their battery monitor (BMV 501)? that will give you lots of information.

Flux

[boB]

You could compare it to a similar size and weight of known lead acid battery maybe

to get a rough idea.

boB

[Flux]

Yes that would be a good starting point, if you can find details of similar sized traction batteries that would probably save you a lot of effort, they seem to be similar if you compare like constructions.

If your figures give results drastically lower then they are likely to be on their last legs.

Flux