Deep cycle battery testing

[Bowie69]

I know there's some battery nerds on here, so wanted to get an idea of what is going on with my 75Ah deep cycle battery, which I am fairly certain needs replacing (~6 years old), but also so there's something on here to help others with questions.

It has a small solar panel attached with proper charge controller which has kept it well topped up over the years, and has never been discharged to flat, with only light usage for lights, water pump, charging phone occasionally, so should be in good condition in that respect, if it is dead, then it is more likely age-related rather than abuse. Impetus for this testing was I'm going compressor fridge due to the price rises in bottled gas, and the temperamental nature of the previous 3-way fridge, which has been very good in the past, but in recent years performance on gas has dropped despite hours and hours of tinkering. It's likely I need a bigger battery to go with the new 100W solar panel, but if I could get away without it, then fantastic (but doubtful...)

So on to the test.... I gave it a full charge, and then after a bit of a rest, connected a ~2A load to it (couple of 2D fluorescent lamps) , measured the voltage roughly every hour, with load and without. The no-load voltage is only allowing it to stabilise for a short time (30 secs or so), so not a true 'at rest' figure which needs 2+ hours to be accurate, but is an indicator, I think.

I did a couple of true at rest figures, and found them to be no more than 0.1-0.2v higher than my measured no-load voltage.

So here comes the graph, which may entertain some of you!

I 

 

All went well for ~ 15 hours, then sudden and catastrophic drop in on-load voltage, however the no-load recovered quite well until at 18 hours on-load voltage dropped so much the lamps turned off.

So.... I am trying to take some conclusions from this, which may or may not be correct, I'm an electrical/electronic engineer by training way back when, but by no means a battery expert

So, can we say:

• As the no-load voltage has dropped to 11.5V at 16 hours, the battery is essentially flat, any more and risk serious damage (as per chart attached).
• If flat at 16 hours with ~2A draw, does that equate the battery capacity to just 32Ah?  (i.e half what it should be)

And some questions:

• Is it kippered? 
• What has kippered it? 
• Is it just old age, like most things?

 

Thanks

 

Peter.

 

 

 

 

 

 

 

 

 

 

 

[smallfry]

No battery expert either, but as it appears you have nothing to lose, (except another £100) I would try giving it some exercise.

Charge it fully and put  something on it pulls about 10 amps, headlamps ? and let it drop to 11.5 volts or so, and repeat. Might clean the plates a bit.

These sort of batteries do like to be used ......... charge and discharge.

I know what you mean about bottled gas, couldnt believe how much they wanted for my little plumbing cylinder !

[Bowie69]

Giving it a bit of exercise today as it happens, after a full charge I got the fridge running off it with no solar input, just for some punishment  

Meant to say, have already run it on a 'repair' mode on a charger, unsure if made any difference, but was worth a try, I guess.

 

[TSD]

Your test method looks valid, and similar to what @FridgeFreezer and I have used previously. Measuring the rest voltage after 30 secs or so is good enough to show the trend, and the effective internal resistance of the battery at each test. I don't think there's much real benefit in waiting longer, other than to standardise the test method. The terminal voltage measured and the equivalent SoC depends on so many factors (battery chemistry and design, voltmeter accuracy, ambient temperature etc.) that there's little point in trying to get a better measurement, or to interpret it in any finer grain.

So, yes, 32Ah delivered capacity. Does your small solar panel/charge controller just deliver a stabilised trickle charge, or does it do something smarter. Low capacity seems the sort of typical failure for standby batteries on permanent float charge.

I'd hope that a few charge/discharge cycles with a high power, reasonably smart charger (ctek or similar) would get some delivered capacity back, maybe to 50Ah or so, which might be enough to get some more useful working life from it? (I got a 50Ah Optima back from 17Ah to ~34Ah with similar treatment). Think of it this way, a 50Ah battery that's free (because you already own it) and you aren't worried about abusing (because it's already bin-fodder), is pretty much as useful as a 75Ah battery that you have to disconnect before it's flat because you don't want to damage it.

 

[Bowie69]

6 minutes ago, TSD said:

So, yes, 32Ah delivered capacity.

Thanks, that makes sense.

 

6 minutes ago, TSD said:

Does your small solar panel/charge controller just deliver a stabilised trickle charge, or does it do something smarter.

Hmm, suspect mostly just a float charge, though it would turn off when the sun goes out of course! Think it cut off at a certain voltage, but can't honestly remember! I have a better MPPT controller coming with the 100W panel, which will be interesting.

 

8 minutes ago, TSD said:

I'd hope that a few charge/discharge cycles with a high power, reasonably smart charger (ctek or similar) would get some delivered capacity back, maybe to 50Ah or so,

Interesting... Sounds like I should continue to abuse it a little and then give a good smart charge/repair charge a few more times and then re-test. Have a similar-to-Ctek charger, so here's hoping. 50Ah would be great to achieve, and as you say free capacity.

 

10 minutes ago, TSD said:

Think of it this way, a 50Ah battery that's free (because you already own it) and you aren't worried about abusing (because it's already bin-fodder), is pretty much as useful as a 75Ah battery that you have to disconnect before it's flat because you don't want to damage it.

 

Like that way of thinking! It's always been light-load previously, so led a pretty cosseted life, perhaps I'll still be asking too much of the poor thing running a fridge overnight, but guess I can try it and see. If not, new battery time.

 

Cheers for the input.

[FridgeFreezer]

Depends how many watts your "small" solar panel is, our 100W panels will peak at about 5A in strong sun, which is a reasonable charge (10A total) into a 100Ah battery but still not much compared to what the alternator can do, or what TSD can inflict on one when left to his own devices with a laboratory power supply

I've got an Optimate charger that has a "desulphation" cycle and I've seen it bring dead batteries back good enough to run the fridge for a fair while, although I'm not sure I really trust a weak battery as my starter.

I'd try giving your battery a few cycles of abuse and a strong charge from the alternator or similar if you can.

[Bowie69]

Yeah only 20W, was a bit skint at the time, but it was calculated as being enough for the little stuff I was doing with it, not enough for a fridge of course, hence the new 100W on the way, which has apparently given 75-80W in testing, so fingers crossed...

 

 

 

 

 

 

 

[Bowie69]

1 hour ago, FridgeFreezer said:

I've got an Optimate charger that has a "desulphation" cycle

Yep, my charger has as well, though not optimate. No worries about using this as a starter, purely fridge duties! Also not going to Mongolia, so never likely far from somewhere I could pick up a new one....

Fridge has been running since 9am, and still going with resting voltage of ~12.1V, the 2 * 1A lamps took just 10.5 hours to get to that. Suspicious that 1A lights (though rated 16W........?!?!) may take more, so perhaps I was over cautious in my estimate yesterday -stupidly I left my multimeter at the workshop so I had nothing here to actually test things.... one arriving tomorrow courtesy of Amazon.

Can't really hook it up to the alternator, as it is sitting in a folding camper But the mains charger seems to deal with it quite well (6A) and charged it to full overnight quite happily, including all the desulphation routine, will probably run it again tonight.

Thanks chaps  

 

 

[FridgeFreezer]

57 minutes ago, Bowie69 said:

hence the new 100W on the way, which has apparently given 75-80W in testing

That feels optimistic unless you angle it just right on a very sunny day nearer the equator than we are - as I said, our 100W panels flat on the roof give out 3-4A reliably on a sunny day, 1-2A on a cloudy day, and they've topped 5A in sunny spots of Europe when it's been blazing hot - so 5A @ ~14v = ~70W, but that drops off either side of noon as the angle changes.

Our rule of thumb is a 100W panel will give the equivalent of a continuous 10W over any given 24H period.

[Bowie69]

That's useful, have a 20A controller so worst case I guess I just bolt another panel on!

[FridgeFreezer]

1 hour ago, Bowie69 said:

That's useful, have a 20A controller so worst case I guess I just bolt another panel on!

Sort of, you won't get optimum out of either panel if you run them through one controller - hence why I have 2 panels and 2 cheap MPPT controllers wired in parallel.

[Peaklander]

13 hours ago, Bowie69 said:

Fridge has been running since 9am, and still going with resting voltage of ~12.1V, the 2 * 1A lamps took just 10.5 hours to get to that. Suspicious that 1A lights (though rated 16W........?!?!)

The fridge has a duty cycle too so not a continuous load. Depending where it is sited in your vehicle (don’t know what that is) you can shade the windows or the fridge directly, to reduce the ambient temp and extend the  cycling. 

[Bowie69]

It's one of these, and yes have the curtain closed as will get some part of the day.

https://www.conwayowners.org.uk/cardinalclubman.htm

[Bowie69]

Sorry for short replies, I phone and still getting issues with whitespace!

[TSD]

As Peaklander suggests, the fridge doesn't represent a good load to gauge the condition of the battery. The energy consumed per hour will vary a lot (breeze, cloud cover, sun angle,  number of beers replaced etc.). Your lights will draw fairly constant power, regardless of the ambient darkness level . Measure the current drawn once and you can use that figure for all future calculations.

Of course, what you really need is a 'systems' performance measurement. The best way to do that would be to wait for some representative weather, then sit in the garden all day with a good workshop manual, drinking only beer from the fridge and cooking burgers taken from the icebox. The best bit is you get to call this activity 'Research' On day two, if the weather holds, you can repeat with the fridge relocated to under the van during daylight hours, instead of inside. With the lower ambient temperature and better cooling breeze, you should get a performance boost that offsets the negative effect of easier beer availability.

[Bowie69]

Yep, stuck the fridge on it just to give it something to do yesterday, and get an idea of how long it would last with current capacity, will be hooking the lights back up tomorrow and see where we are again -it's been on a desulfate cycle all night and some of today (in London at the mo), but sure it will be charged by the morning

 

Unfortunately I'm not a drinker, but for everyone else I'm sure the rest sounds lovely!

[Bowie69]

Bit of an update, mostly to say that I tried reviving the old battery, but ultimately there simply wasn't enough in it to run the fridge for more than a day despite many repair cycles, it gained perhaps 5Ah, which was simply not enough. Additionally I think there was something going on where the on-load voltage would drop more than it should, so occasionally the fridge would trip out despite being set to 9V as the minimum.

In the end I bought a standard sized (helps with the price), 110Ah 4 yr leisure battery , which was £97 delivered, and should be MORE than enough to keep the fridge going with the solar.

Current solar set up is 2*100W panels, 20W smart MPPT controller, which seems to have a good bit of spare capacity even on overcast days like today. I have been running a compressor fridge off it for about a week now, and never has the on-load voltage dropped below 12, (so allowing the battery to recover would likely mean it is actually 12.2V, and therefore at least 60% charged).

The fridge side is running at ~4C, and the freezer set to -12C (though it could go to -18C, I don't see the need). I don't open it often, nor reload with warm stuff, so this should be considered only a baseline set of figures, perhaps add 25-50% depending on what at you are asking it to do?

The smart charge controller load output is logging ~330-375WhWh (between 26 and 30Ah) per day power use. 

Power generation is normally around 400-475Wh per day, regardless of clouds, I guess it can only log power it can use, so this could well be higher.

The battery is consuming ~30-35Ah per day to replenish the charge, this is slightly higher than the load consumption, due to charging losses, I assume.

Using the figures above I could probably run the fridge without any solar at all for 3 days off the 105Ah battery (though would be very low, and potentially damaging to the battery), but it would do it. The solar more than keeps up in testing, I do wonder now if I could have got away with one 100W panel, but actually, having the second one allows more capacity for other things like charging phones/other devices I may choose to add, so all good.

Feedback welcome, if there is any....

 

 

 

 

[cackshifter]

Very interesting to get some real world actual experience numbers. 

[Bowie69]

Also, now I remember, the 200W of panels have managed 166W peak power generation, so pretty happy with that, given our location on the globe, and them being flat.

[Peaklander]

You have good data, better than me. My controller tells me nada. I have a Hall effect sensor so can see net current flow into the battery and voltage of course. When away, as now, I wish I had my clamp meter and then I could look a little closer.

I have a 100W panel and one leisure battery. I suspect that in my case two panels would be better than another battery.  I think it’s generation rather than storage that I need more of.

The panel orientation is critical and with a lifting roof this can be a hindrance as well as a help. 

[Bowie69]

Cheers, yes, theres an app for my phone to pull off historical data, seems to be logged ad infinitum.

The kit I went for was a Renogy 100W MPPT:

https://www.amazon.co.uk/gp/aw/d/B06WGW485F?psc=1&ref=ppx_pop_mob_b_asin_title

I added the Bluetooth module so I could be less in the dark (plus the controller is under a seat in the camper....), and pretty surprised/impressed by the level of logging available.

The charge controller when sold alone seems quite pricey, but with the kit, and a discount code at the time, it was good value, and pretty good quality. Panle mounting brackets weren't ideal, but got the job done, plus have you seen how much some people charge for them alone?

[Aragorn]

Keep a close eye on it when the weather is poor.

We have solar on the house (notionally 3800w), on a sunny day it'll pump out 2500w peak, sometimes higher in short bursts. On a cloudy rainy day in winter, it often wont get past 200w. Vast difference in output. Even taking this week for example, i managed to generate a total of 16kwh on Friday, but only 3kwh on Monday.

Your system appears to have plenty of capacity, but a few consecutive days of poor weather might well mean the panels stop keeping up with the fridge. Luckily, your in Somerset, so probably have less cloudy rainy ming than we manage up here in fife 😆

[Bowie69]

Thanks Aragorn, I will come back to you on that, as had some pretty miserable weather down here recently , and got some good data