Accurate Battery Testing.

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doug748

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I have now fitted my Nasa monitor and very fine it is.

It strikes me I could now do a pretty accurate battery test, if I knew anything about electricity.

If I took a fully charged nominal 100Ah battery and, say, left a 25w bulb on for 10 hours; what sort of readings should I be seeing on the monitor?
What are the minimum levels before I should think about binning the battery?
What would be the resting voltage after the test?

I can, of course take the temperature of the battery if needed.

Thanks for all suggestions on this one.
 
doug748 said:
I have now fitted my Nasa monitor and very fine it is.

It strikes me I could now do a pretty accurate battery test, if I knew anything about electricity.

A. If I took a fully charged nominal 100Ah battery and, say, left a 25w bulb on for 10 hours; what sort of readings should I be seeing on the monitor?
B. What are the minimum levels before I should think about binning the battery?
C. What would be the resting voltage after the test?
Click to expand...

A. 25W at a nominal 12V is 2A, near as makes no difference. So after 10 hours, 2A x 10hrs = 20Ah. Assuming 100Ah battery truly was 100% full before test (really need to check with hydrometer), that would put it at 80% charged. For this you'd expect a reading of around 12.5V (after rest) and a hydrometer reading (SG) of around 1.238. SG of a fully charged battery should be around 1.277. I've no personal experience of the NASA monitor but presumably it would report accordingly.
B. If the hydrometer readings are significantly different between cells (say more than .03 difference), an equalization charge may be indicated, but only if the batteries are flooded, wet. If the SG readings when fully charged are 1.25 or less, then equalization is also indicated. Batteries should be fully charged and topped-up with water before equalization. If after equalization the SG does not rise above 1.25, then the batteries are probably past their best.
C. around 12.5V, as above.

Note that the usual way to do a capacity test is to introduce a known load (as you have suggested) for a time which would take the batteries down to what should be 50%, then check SG. There's a bit more to it than this since the effective capacity of batteries differs between high and low loads. Some battery monitors have an algorithm to compensate for this effect.
 
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[TABLE="width: 258"]
[TR]
[TD="align: right"]100[/TD]
[TD="align: right"]1250[/TD]
[TD="align: right"]1240[/TD]
[TD="align: right"]12.75[/TD]
[/TR]
[TR]
[TD="align: right"]90[/TD]
[TD="align: right"]1235[/TD]
[TD="align: right"]1225[/TD]
[TD="align: right"]12.65[/TD]
[/TR]
[TR]
[TD="align: right"]80[/TD]
[TD="align: right"]1220[/TD]
[TD="align: right"]1210[/TD]
[TD="align: right"]12.55[/TD]
[/TR]
[TR]
[TD="align: right"]70[/TD]
[TD="align: right"]1205[/TD]
[TD="align: right"]1195[/TD]
[TD="align: right"]12.45[/TD]
[/TR]
[TR]
[TD="align: right"]60[/TD]
[TD="align: right"]1190[/TD]
[TD="align: right"]1180[/TD]
[TD="align: right"]12.35[/TD]
[/TR]
[TR]
[TD="align: right"]50[/TD]
[TD="align: right"]1175[/TD]
[TD="align: right"]1165[/TD]
[TD="align: right"]12.25[/TD]
[/TR]
[TR]
[TD="align: right"]40[/TD]
[TD="align: right"]1160[/TD]
[TD="align: right"]1150[/TD]
[TD="align: right"]12.1[/TD]
[/TR]
[TR]
[TD="align: right"]30[/TD]
[TD="align: right"]1145[/TD]
[TD="align: right"]1135[/TD]
[TD="align: right"]11.95[/TD]
[/TR]
[TR]
[TD="align: right"]20[/TD]
[TD="align: right"]1130[/TD]
[TD="align: right"]1120[/TD]
[TD="align: right"]11.85[/TD]
[/TR]
[TR]
[TD="align: right"]10[/TD]
[TD="align: right"]1115[/TD]
[TD="align: right"]1105[/TD]
[TD="align: right"]11.75[/TD]
[/TR]
[TR]
[TD="align: right"]0[/TD]
[TD="align: right"]1100[/TD]
[TD="align: right"]1090[/TD]
[TD="align: right"]11.65[/TD]
[/TR]
[/TABLE]


% Specific gravity range volts
 
Sailorman: not quibbling in the slightest, but interested in where you SG figures come from. 'Standards' do seem to vary. I have Trojan batteries so use the figures provided by Trojan (which also happen to agree with my hydrometer, which is reassuring).
I certainly agree with your implication that checking SG is key. In fact that's just what I'll be doing after finishing this coffee...
 
Just info i was given by another forumite :encouragement:

all my batteries are sealed as they are abaft the engine & a pig to get at

PS i too have just fitted a Nasa BM 2. take the % remaining with a pinch of salt
 
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You can measure open circuit voltages and s.g.'s until you're blue in the face but the only certain way of verifying a battery's capacity to a particular specification is to test it to that specification. OCV and sg will give an indication of state of charge in a battery that is in good condition, but it cannot tell you whether the battery is in fact in good condition. It will not show effects of some problem conditions e.g. high internal resistance.
OCV and sg against state of charge will vary for different batteries, according to fully charged s.g. (can be anything from 1.220 to 1.300 although 1.280 is pretty common in the sort of batteries under consideration here) and fully discharged s.g. (typically 1.100-1.200 for flooded, can go lower for AGM.) OCV per cell = (sg + 0.85)V at 25 deg C, this is pretty accurate but not absolutely precise especially as s.g gets low. So at 1.280 expect 12.78V from a 12V battery, at 1.100 expect 11.7V. Note that OCV is related to s.g., not directly to state of charge. Measuring OCV is just a proxy for measuring s.g.
Doing a discharge test accurately is not easy.
In test labs it's done under temperature controlled conditions with computer controlled equipment that maintains a constant current load despite voltage variation (or a constant power load if the battery has been specified that way i.e. capacity in Wh instead of Ah) with continuous monitoring, down to the specified end voltage under load, e.g 20 hours to 1.7 volts per cell.
Fail (for critical applications) is normally taken as 80% i.e. if it reaches the specified end voltage in 80% of the specified time.
If you want to do it with simpler equipment e.g. a light bulb as the load you need to monitor current as well as voltage as it won't be constant. You can then plot it out and integrate for Ah. You'll also need to compensate for temperature.
 
doug748 said:
..... if I knew anything about electricity.....
Click to expand...

doug748 said:
Just one question macd, what is equalization?

Apart from that I am set to go, thanks.
Click to expand...


I'm not sure where you think you are set to go?

As stated measuring the Ah capacity with discharge tests is very very difficult.

Just to make it clear these are the things you need to understand.

1. The current Ah capacity of the bank to work out the 20 hour discharge rate current that you need for the test. You don't know this until you've done the test!
2. A way to keep this discharge current CONSTANT for 20 hours, as the voltage goes down the current and therefore the Ah discharged changes.
3. You have to discharge to a voltage of 10.5v which batteries don't like too often.
4. You can't to a discharge test to only 50% SOC because you don't know the battery voltage at the 20 hour discharge rate, because all OCV or Load voltage change with the age of the battery.
5. If you do the test and get an approximate remaining Ah capacity of say 80% of when new then you need to repeat the test with a 20 hour discharge rate that is 20% of what you used in No 1 above.
6. You need to do the test when the battery are at 100% full - that's not when the float light comes on.
7. Equalise the batteries if they are open wet lead acid, or Lifeline or Red River AGMs.

A very very expensive Capacity test instrument can do the job, but a cheap starter battery drop tester is no good on deep cycle house batteries.

Electrickery is not easy.
 
Agree with SailingLegend420 (as usual!) except for point 1.

The actual present (avoid confusion with word "current"!) capacity is not relevant, you don't want to test to that.
It is only meaningful to test and evaluate against the nominal capacity, which you presumably do have.
So if it's a 100Ah battery and we assume it is rated on the 20hr capacity (not 3hr, 8hr, 10hr or 12hr which are all alternatives in different standards) you want a test current of 5A.
You can approximate to this with 60W headlamp bulb BUT the current won't be constant.
If it's a 20hr rating the most likely end voltage is 10.2V - but there are alternatives to that too. Could be 10.5V.
Going to that level once won't hurt it but you must recharge immediately.

Point 5 is technically correct but I would suggest not worth worrying about. In industrial testing if it doesn't achieve 80% of the time at the nominal current, it's junk. There's no point trying to get an accurate 20hr current capability, and without the right equipment you aren't going to achieve that accuracy anyway. There will be little difference between a 16hr and a 20hr capacity in Ah. You would only go to all that trouble in characterising test samples of a new battery design, not passing or failing a battery in service.

Equalising charge? After repeated cycling, particularly if not fully charged every time, cells in a battery get a bit out of step as measured by OCV or s.g. (or capacity). Remedy is a vigorous charge. Traditionally that would be done at 5% or 10% constant current for some hours, which can produce pretty high voltages. Modern chargers aren't usually capable of that so you do it at constant voltage typically around 14.5V, some batteries specify up to 15.5V - which few chargers will do.

Incidentally don't underrate point 6 - it's not that easy to know when a battery is fully charged. When your multi stage charger switches to float it can be well below full charge, easily 10%. More batteries fail from undercharge than overcharge!

sailinglegend420 said:
I'm not sure where you think you are set to go?

As stated measuring the Ah capacity with discharge tests is very very difficult.

Just to make it clear these are the things you need to understand.

1. The current Ah capacity of the bank to work out the 20 hour discharge rate current that you need for the test. You don't know this until you've done the test!
2. A way to keep this discharge current CONSTANT for 20 hours, as the voltage goes down the current and therefore the Ah discharged changes.
3. You have to discharge to a voltage of 10.5v which batteries don't like too often.
4. You can't to a discharge test to only 50% SOC because you don't know the battery voltage at the 20 hour discharge rate, because all OCV or Load voltage change with the age of the battery.
5. If you do the test and get an approximate remaining Ah capacity of say 80% of when new then you need to repeat the test with a 20 hour discharge rate that is 20% of what you used in No 1 above.
6. You need to do the test when the battery are at 100% full - that's not when the float light comes on.
7. Equalise the batteries if they are open wet lead acid, or Lifeline or Red River AGMs.

A very very expensive Capacity test instrument can do the job, but a cheap starter battery drop tester is no good on deep cycle house batteries.

Electrickery is not easy.
Click to expand...
 
doug748 said:
Just one question macd, what is equalization?
Click to expand...

It's a controlled charge at 15-odd volts and an amperage proportional to battery capacity. Putting it very crudely, it 'zaps' the batteries back to spec (reduces stratification, etc, etc, I'm sure Wiki can tell you more). It's suitable only for open (i.e. top-uppable) lead-acid batteries, and evidently some AGM batteries. If your battery charger (or some similar kit) doesn't have an equalization facility, then you can't easily do it.

I might add that what you propose may be useful but is a poor man's battery test, largely for the reasons I suggested and above all for those proposed by other posters more learned than me. But your curiosity will get you far (does that sound patronising?: it's not meant to).

P.S. I was typing as Plevier submitted his post, which I suspect already answered your question.
 
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Plevier said:
Agree with SailingLegend420 (as usual!) except for point 1.

The actual present (avoid confusion with word "current"!) capacity is not relevant, you don't want to test to that.
It is only meaningful to test and evaluate against the nominal capacity, which you presumably do have.
So if it's a 100Ah battery and we assume it is rated on the 20hr capacity (not 3hr, 8hr, 10hr or 12hr which are all alternatives in different standards) you want a test current of 5A.
Click to expand...
What I meant was you can't take the nominal capacity if the battery is old - you do need to use the actual present value (Fully Charged) otherwise your 20 hour test is not valid - which is why you may need to repeat the whole process. So your 20 hour rate on your 100ah battery - that is now only 85Ah - would mean a constant current load of 4.25 amps not 5 amps. So reduce the nominal value by 5% a year and use that value if you are masochistic enough to try this test. 80% of nominal value is I think too high to call junk, that's what the battery manufacturers want you to do. Lifeline say I can go down to 50% before my AGMs are junk.

This is all not really worth arguing about because the test is so difficult to do with any kind of accuracy - just put a few amps on for a few hours and see what the voltage falls to! Do this when the batteries are new and repeat every year and plot the results on a graph. You can immediately see if there is a difference, but you still won't know what Ah capacity you have lost!

The answer is to "know your boat" - if your battery voltage was above 12.2v with no load after 24 hours - approx down to 50% SoC - and now it only takes 12 hours then either you have an extra current drain or the batteries are losing their Ah capacity.

A Battery Monitor - reset manually when 100% - or better still a SmartGauge will give you a better idea of what is happening.
 
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sailinglegend420 said:
I'm not sure where you think you are set to go?
Click to expand...


Well, it is obviously a complex field. My proposed little test may not give a valid prediction.

But:

I know how the battery should perform when new. I know how it has performed in the past. I know the minimum 24hour performance I require.

So I will just do a real time, full load, test and it will either cut the mustard or not. If it flakes out too early I will get a new one. It will also be handy to see how the Nasa monitor plots it's decline.
 
I wouldn't take the voltage shown on your NASA BM as accurate. My BM1 consistently shows 0.2v more than a digital Multi Meter backed up by the boats built in monitor.

John
 
I have a battery conductance tester that gives a direct readout of the Cold Cranking Amps available. I've found that a comparison with rated CCA or historical readings is a very good indication of battery condition.
 
A battery's capacity is a very rubbery number given by the manufacturer under the very best new condition and then with added optimism. However it does give an inmdication of relative size. So if you can get 50% of the claimed AH rating on a test you are doing really well. Though you should not do this test or at least not often as deep discharge is bad for them.
In the final analysis a battery is good if it does the job you need of it. So firstly will it (alone) crank the engine with enthusiasm when cold. if so it is good for cranking.
Will be battery appear to hold up OK with your overnight demand when camping or sailing. You can use a battery monitor to check actual usage here. Monitor voltage to make sure you don't discharge too much.
In practice you will know if a battery is no good and if it appears OK why worry. Batteries can die quickly and unexpectedly for engine starting. The decline of capacity is perhaps more gradual. One thing for sure you cannot accurately estimate the health of a battery or the amount of power left in it. A BM really only rells you what you are putting in or taking out of a battery. good luck olewill
 
savageseadog said:
I have a battery conductance tester that gives a direct readout of the Cold Cranking Amps available. I've found that a comparison with rated CCA or historical readings is a very good indication of battery condition.
Click to expand...
That might be a little optimistic...

From here.. http://batteryuniversity.com/learn/article/how_to_measure_capacity
meas2.jpg
 
savageseadog said:
I have a battery conductance tester that gives a direct readout of the Cold Cranking Amps available. I've found that a comparison with rated CCA or historical readings is a very good indication of battery condition.
Click to expand...

Hi,
Which tester have you been using and what is the verdict?
I have been thinking about getting either a Cen Tech www.amazon.com/Cen-Tech-Digital-Automotive-Battery-Analyzer/dp/B005RGMFQE
Or an Augocom Micro-468

I would appreciate any feedback.

Thanks,
Findus
 
findus said:
Hi,
Which tester have you been using and what is the verdict?
I have been thinking about getting either a Cen Tech www.amazon.com/Cen-Tech-Digital-Automotive-Battery-Analyzer/dp/B005RGMFQE
Or an Augocom Micro-468

I would appreciate any feedback.

Thanks,
Findus
Click to expand...

If you want to check the battery's starting capability it will give you an idea, but if you want to know reliably the low rate capacity (i.e. anything longer than minutes) forget it. It's governed by different characteristics.
 
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