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Old 20th Jan 2019, 10:58 pm   #1
ITAM805
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Default Doable or dumb?

Hi all

I have a 48V 2kW solar setup and recently one of the 12V lead acid batteries died. So I replaced it, but with a different type (Varta 60Ah) from the remaining 3 'Powerline' 65Ah. I should say I've found the Powerline batteries to be crap

Now I know the rule is the batteries should be the same voltage for balanced charging, and to aid this I fitted each battery with a circuit (green) that turns on a dummy load when the charge reaches 13.8V and this worked well, that is until I fitted the superior Varta, which doesn't reach 13.8V and thus is undercharging.

So, is it feasible to charge the Varta (magenta) from an isolated charger (orange) whilst 'in circuit', that is while the battery string in charged from the 48V source?
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Old 20th Jan 2019, 11:42 pm   #2
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Default Re: Doable or dumb?

Sorry I should have stated the charging voltage is 55.2V (13.8 x 4)
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Old 21st Jan 2019, 12:21 am   #3
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Default Re: Doable or dumb?

I don't understand how a shunt regulator to limit battery voltage to 13.8V can cause the battery it's across to be undercharging. Surely if the Varta isn't reaching 13.8V at full capacity it would be overcharging. Could it simply be that the Varta battery has more actual capacity than tired Powerline ones so it's just taking longer to charge?

It also seems very odd that it won't reach that voltage- for a one shot charge you would normally run constant current at about 14.4V limit until the charge current falls to around 10% of its initial value after the battery hits 14.4V then drop back to 13.8V to float the battery. No problem with exceeding 13.8V on the way up.

What does the Varta battery datasheet have to say about recommended charge routines?
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Old 21st Jan 2019, 12:38 am   #4
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Default Re: Doable or dumb?

Quote:
Could it simply be that the Varta battery has more actual capacity than tired Powerline ones so it's just taking longer to charge?
I think that's the issue Chris, it's not reaching 13.8V while the Powerline do quite quickly, after an hour or so. It hasn't been sunny here generally which hasn't helped! So I was wondering if I could give the Varta a leg up?

I haven't read the specs on the Varta but the Powerlines recommend charging at 13.8V and a single 14.4V charge once a month
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Old 21st Jan 2019, 1:25 am   #5
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Default Re: Doable or dumb?

You could buy three more Vartas. It sounds lik you're going to need to do so quite soon.

David
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Old 21st Jan 2019, 2:45 am   #6
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Default Re: Doable or dumb?

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Originally Posted by Radio Wrangler View Post
You could buy three more Vartas. It sounds lik you're going to need to do so quite soon.

David
Have to agree. Hard to fork out for the extra I know but true I'm afraid...

Steve.
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Old 21st Jan 2019, 9:48 am   #7
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Default Re: Doable or dumb?

I see no problem "jumping" the Varta, it will seem to the rest of the system as going up in volts a bit.
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Old 21st Jan 2019, 11:34 am   #8
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Default Re: Doable or dumb?

Yes, you could also charge the Varta offline then add it to the string. The "hole" in the string will be filled in by the bypass unit anyway to keep the rest happy whilst charging, though you won't be able to take much power! Does your bypass have a reverse current protection diode?

13.8V charge is really just good for floating a fully charged battery indefinitely, to get a full charge at that voltage from empty will take a loooooooong time for the last bit. That's likely why Powerline suggest a regular "topping" charge since you're not usually just floating a standby battery in a solar PV application, you're running in a part cyclic mode. Lead acids are not happy for any significant time in anything other than a fully charged state.
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Old 21st Jan 2019, 2:23 pm   #9
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Default Re: Doable or dumb?

Thanks guys

Merlin, that's the answer I needed

Yes of course the definite cure would be to install 3 new Varta's, but I'm trying to cheapskate my way out of this, at least temporarily putting off the inevitable
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Old 21st Jan 2019, 2:43 pm   #10
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Default Re: Doable or dumb?

Swapping to all Vartas now will at least mean you have a reasonably matched set into the future.

If the Powerline batteries are on the way out, you'll need to keep rebalancing things repeatedly.

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Old 21st Jan 2019, 5:26 pm   #11
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Default Re: Doable or dumb?

Just to throw something in from "left field", my understanding is that the main failure mode for batteries in these systems is sulphation, and it is the solar power community that led the way to the use of de-sulphator units.

I've owned one of these for several years now, and I'm quite convinced that they do have a good effect on old sulphated batteries. Of course, there are other factors that cause batteries to decline, especially in car use, and de-sulphators are of limited benefit in those cases.

The de-sulphator which I have came from this company http://www.courtiestown.co.uk/. The fact that they have been in business so long, says something about de-sulphators for those who are doubtful (no connection other than satisfied customer)

I understand that the International Union of Pure and Applied Chemists (IUPAC) has now agreed that everyone should use the term sulfation, but I don't want to do that... and also, I still regard Pluto as a planet.

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Old 21st Jan 2019, 7:10 pm   #12
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Default Re: Doable or dumb?

Batteries from different manufacturers may have different 'chemistries' which will determine the terminal voltage and charging voltages.

I would imagine that even batteries from the same batch of a production run from one manufacturer would have slightly different terminal voltages and Amp hour capacities.

Hopefully you already have an intelligent charger that will limit the current and both the fast charge and float charge voltages.

A balancing charge will be required occasionally.

If I were in your situation, I would be tempted to buy four cheap LED voltmeters and put one across each battery.
You could then monitor the voltage of each battery during charging and discharging.

I would not replace the Powerline batteries until I saw signs of age or failure.

Last edited by Silicon; 21st Jan 2019 at 7:14 pm. Reason: More comment.
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Old 21st Jan 2019, 11:27 pm   #13
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Default Re: Doable or dumb?

Quote:
Originally Posted by Silicon View Post
Batteries from different manufacturers may have different 'chemistries' which will determine the terminal voltage and charging voltages.

I would imagine that even batteries from the same batch of a production run from one manufacturer would have slightly different terminal voltages and Amp hour capacities.
I suspect that it is true that even for lead acid batteries coming from the same production line, that the length of time they were then in storage prior to being sold could cause small changes in behaviour, but those may be magnified over the period of the battery's working life unless the charging system senses each battery separately.

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Old 22nd Jan 2019, 3:04 am   #14
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Default Re: Doable or dumb?

Quote:
Originally Posted by ITAM805 View Post
Yes of course the definite cure would be to install 3 new Varta's, but I'm trying to cheapskate my way out of this, at least temporarily putting off the inevitable
Now you know that's just going to cost you more in the long run .....
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Old 22nd Jan 2019, 3:19 am   #15
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Default Re: Doable or dumb?

Cheapskating wont work !!!

After 35 years of doing solar power, I can guarantee that you will end up with a dead battery bank. The three Powerline batteries will shortly fail. You will buy three new Varta's to replace them, ONLY to find the Varta fitted now no longer matches the three new ones.
Second set of batteries will also be a failure!!.
TRy putting 3 off 6 inch wide tyres on your car, and a single 5 inch tyre. Its only a "small" discrepancy.
60 Ah and 65 Ah is also only a small discrepancy. Internal resistance of the new battery will be MUCH different to the old ones.

Joe
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