battery capacity test

[davehu]

This is one for the battery experts. I have two 80 AH domestic batteries, both are 6 years old and probably need rheplacing but I thought I would find out what condition they are in. They Hold their charge well but seem to lack capacity, probably a sympton of getting old, The batteries not me.

I charged them up at home to an at rest reading of 12.57 volts which equates to 93% when adjusted to 10 deg C. I applied a load of 42 wattts (3.5 amps) for a total of 7.8 hours in two stints allowing about 5 hours in between. At the end of time after allowing time for the battery to stabilise the off load volts was 12 .00 volts which corrected to 10 deg C equates to 50% Charge.

so Amp hours consumed was 3.5 x 7.8 = 27.3 AH

percent of battery used 93 - 50 = 43%

Hence total capacity is 63.5 AH instead of 80 AH.

Is this a fair calculation, if so it explains why the cappacity seems to be down nad why is charges up quicker than i would expect.

Your comments please.

 

[oldharry]

Sounds fair. Get your garage to do a heavy discharge test: this will show upany shortcomings. A fully charged battery should deliver a specified current in the order 300 amps or more for a specific time, without dropping below a specified voltage. If capacity is down, the voltage will fall below spec before the end of the test. The amperage required for this test should be specified on the battery label, sometimes as 'cranking capacity' and can be as high as 500a or more for a big diesel starter battery

 

[cliffordpope]

The best way to test a battery is to try and use it in the way you would like, and see if it comes up to scratch.
Charge up the battery, leave it for a month, try and start the engine. Yes, no, almost, once only?

 

[Pye_End]

What sort of charger did you use?

Unless you use a 3 stage charger you will not gte to 100%

Also don't rely on a voltage reading when just charged - take a little off it first.

Always worth checking the SG of the cells (if you can get to them) when you have just charged up.

 

[Guest]

A garage test surely ....

If you were to leave a Garage Load tester on a battery to check out Colc Cranking Amps time / claim - surely the battery plates would buckle with the load applied ?

The load tester at a garage is there to do an instant short to see if battery has dud cell etc. and should never be left for anything more than a quick dab ???

I think I would prefer the car headlight or other smaller load applied watching the time and voltage ..... or as another says .... charge it up ... leave it for a while - then start engine ..

 

[pohopetch]

If these domestic batteries are true deep cycle batteries here are a few things to consider when testing them:

They won't have a CCA rating and doing a discharge test at a very high rate (e.g. the garage tester) could damage them as they are not made as starting batteries for very high currents (more than 300A).

Their amp hour rating of 80 Ah would be calculated at a certain discharge rate - usually the 20 hour discharge rate. So to do a true comparison of the current capacity with the rated capacity you should discharge them at 4 Amps for your test. A lower discharge rate will give a higher Ahr result, and a higher discharge rate would give a lower Ahr result. The test should not have the 5 hour gap in the middle either, and should go all the way down to 10.5V under load (not resting voltage) from fully charged. If the battery is in good condition it will recover very quickly up to about 11.8V or more when you remove the test load.

The test load will need to be such that it keeps the current up as the battery voltage drops. A lamp will not do this as the current will drop as the battery voltage drops. Using a small inverter running some 230V lights is better, but in fact the power wil stay constant and the current will rise as the voltage drops. If you use a dimmer on the lights you can adjust them down slightly to keep the current constant on your test meter. But this might be going over the top for a simple test!

Some deep cycle batteries have a "reserve capacity" rating which is usually the number of minutes they can be discharged at 25 Amps.

If the battery is one of the "leisure" type is is probably not true deep cycle and you probably shouldn't discharge it all the way down to 10.5V under load as the plates will be thinner.

Your results as given don't look too bad though. Whenever I've tested a deep cycle battery even only a year old I've never got the rated capacity. A good true deep cycle battery will easily last 7 to 10 years if kept charged, watered etc. I've seen some large capacity batteries on solar power installations last 15 years.

 

[Guest]

Deep cycle / leisure batterys ... plate thickness ....

Surely these are the ones with the THICKER plates to withstand the discharge lower than 40 - 50% that a car normal thinner plate battery is only good to.

eg Alastair Garrods excellent book Electrics Afloat ..... page 30 + 31 .....

Basically car batterys with many thin plates to give high surface area contact and therefore short high bursts of amps. Deep Cycle / Leisure batterys with fewer thicker plates with less surface area providing less high bursts of power but longer discharge to lower voltage levels ...

 

[pohopetch]

Re: Deep cycle / leisure batterys ... plate thickness ....

Yep - possibly I wasn't clear there - starting batteries have thin plates with more surface area, true deep cycle have thick plates and leisure something in between. So the leisure battery could be damaged by a real deep cycle down to 10.5V, or at least lessen its life. The deep cycle can easily give say 100A on an L16 battery, but its internal resistence and lack of surface area will mean it can't give say 500A like a start battery. The internal resistance means that as you try to draw a very high current the voltage loss across the battery itself becomes significant, and you will get much less voltage at your load - down to less than the minimum of 9.5V under high load that a start battery is rated at.

 

[William_H]

Re: Deep cycle / leisure batterys ... plate thickness ....

I think you are worrying too much. You have the luxury of a dual battery system so if you handle the system correctly you will know when one is dud and you will still have an alternative.
So frequently start on one battery alone and alternate frequently to the other battery. Discharge batteries for domestic use one at a time so roughly compare their capacity. If one drops dead it will be obvious and you will ahve a standby.
As for capacity in aviation when calculating a survival time on a lead acid battery with alternator dead we only ever expected 50% of rated capacity to be available down to a voltage where radios etc wouldn't work. (proably closer to 10 volts) So if you got 27 AH out of your 80AH battery and the voltage was still at 12 volts you were doing pretty well. ie batts are good provided they will crank engine alone and with usual enthusiasm. olewill

 

[Guest]

Re: Deep cycle / leisure batterys ... plate thickness ....

And of course as I found !! Putting a high load on a Leisure / Deep Cycle battery buckles the plates ... as it cannot take the load ....

 

[Nuvalec]

Re: Deep cycle / leisure batterys ... plate thickness ....

There was a company based up near st helens that did a battery tester which actually measured the amp hour capacity of the battery, but I can't seem to find their website anymore /forums/images/graemlins/frown.gif
Bit off topic, but what happens when you gents decide to buy a lovely big inverter/charger and stick on your deep cycle/leisure batteries? a 3KW sterling, for example, can draw over 280 amps continously...

Tony.

 

[pohopetch]

Re: Deep cycle / leisure batterys ... plate thickness ....

You must have enough amp hour capacity to support the continuous load. A battery bank with amp hour capacity of 4 times the maximum sustained load in amps is one rule of thumb (for 12v systems). So for the 3kw inverter you really need 1200 Ahr of capacity. This is necessary a) to give a reasonable battery life and avoid destroying them in short order b) give a reasonable run-time on the inverter (say 30 mins) c) ensure that the internal voltage drop in the batteries is not so great that the inverter low voltage shut-off does not kick in immediately the load is applied.

If you put your 3kw inverter on at full load on say two 100 Ahr 12v leisure batteries connected in parallel (a typical house battery setup) , you would probably not get more than 1 or 2 minutes from it before the low voltage alarm went off. At 280 amps you might also overheat the batteries and buckle the plates.

Remember that amp hour capacity is stated at the 20 hour discharge rate. If you duscharge at very high rates the capacity is much much less.