Battery charging advice

[CJH]

Long and wordy question coming up, sorry. Just trying to be as detailed as I can.

I have a main starter battery and a leisure battery, with a split charge system that uses a voltage-sensing 'smart' relay. I've adjusted the smart relay so that the batteries are connected once the starter battery voltage rises above 13.4V, and they disconnect again when the voltage drops below 13.1V. This seems to work well with my alternator.

I also have a cheapo LIDL trickle charger (max 4A) which I connect to the starter battery via the mains hook-up when the van's not in use. There's a nice side-effect of the smart split charge relay in this case, in that the trickle charger takes the starter battery voltage above 13.4V, so the relay closes and both batteries then get the benefit of the trickle charger and stay topped up. That's something you don't get with a conventional relay that's triggered by the alternator.

I've had a bad experience with parallel charging lead acid batteries in the past, although I now put that down to the high current output of the charger I was using (>30A), and I can't see that there'll be any problems with this puny 4A charger. But nevertheless, the smart relay almost guarantees that the two batteries will be at dissimilar states of charge, since the relay only closes once the starter battery is reasonably well charged.

So my question for the battery experts out there is twofold:
1) Can you see a problem with the trickle charger in this setup? It's an 'intelligent' charger, and adjusts the current level depending on the voltage of the battery, so when the two batteries are linked and one is charged and the other isn't, then the charger will probably be providing more current than the starter battery needs, but since it's 4A max then I should be ok I guess.

2) What about if I use a higher current 'fit-and-forget' charger, say a 10A charger? Will the charger try to put a full 10A into a fully charged starter battery because the combined starter+leisure battery appears to be below the trickle charge threshold, or doesn't it work that way? Is 10A enough to damage the starter battery in that case?

I have a supplementary question, and if you thought it was complicated so far, just wait.
My van has a ZIG CF8 with an integral 10A charger that connects only to the leisure battery. But of course, when the smart relay closes and joins the two batteries, this 10A charger sees both batteries. If I turn off the trickle charger at this point, then the ZIG charger keeps the starter voltage above 13.1V, so the smart relay stays open and both batteries then get the benefit of the 10A charger. I don't know anything about the ZIG 10A charger, so I don't know how 'intelligent' it is, and I've therefore opted NOT to use it when the van isn't in use. I rely on the trickle charger instead.

So the supplementary question is, are there any potential problems leaving the trickle charger AND the ZIG charger connected to my battery pair?

[ghost123uk]

The short answer to all that is = "you will have no problems"

The only "rider" is that applies as long as your Leisure battery is in reasonable nick. If it ever gets to the state were it is knackered, your charging set up may play up. Personally I would not use 2 chargers (zig plus any other) at the same time, although in theory it should do no harm.

When any type of split charge relay is activated, the batteries in effect become "as one" and any incoming charge (be it alternator, cheap charger, smart charger, via ZIG or not) will be sent the both batteries in a way that does not cause any problems or bad effects

So nothing to worry about there then

[CJH]

Thanks Ghost - that's just what I wanted to hear. I can cope with abnormal situations so long as all that happens is the system 'plays up'. What I want to avoid is a dangerous charging scenario, with a fire risk inside my unattended van. You remember my battery photo, right?



My leisure battery has an LVD attached, so it shouldn't become too discharged. The LVD causes it's own little problem when it disconnects because the wire from the ZIG is both the load wire and the charging wire, so I have to mess about with a jump lead to get the charger reconnected. It adds a level of complexity that's easy to get confused by, but I find that as long as I run it over in my head, say every half an hour, I can remember how it all works!

So based on what you said, if I want to add a solar charger into this setup, can I just connect it to the starter battery and let the smart relay take care of switching in the leisure battery, to give me a sort of 'starter-priority dual solar charger'?

[ghost123uk]

Yeah, that battery pic is uber scary

So based on what you said, if I want to add a solar charger into this setup, can I just connect it to the starter battery and let the smart relay take care of switching in the leisure battery, to give me a sort of 'starter-priority dual solar charger'?

Yep (except it might take a while, depending on available sunlight and state of starter battery, before it climbs to the required 13.xx volts to kick in the smart relay)

My solar panel charge controller has a ciggy plug on the end, so I can charge the starter battery via the front ciggy socket, or leisure via a rear mounted ciggy socket. I did wonder about hard wiring it with a "change over" switch (starter / leisure with live from solar in the centre) might do that today !

Mind you, you say you have a zig panel, could you not wire the solar live to the "car / van" change over switch on that, then you could charge either, no, hang on, if you need to charge the starter battery (via solar, which would be rare occurrence when camping), then the switch would have to be set to "car" which in turn means your accessories would then be running off the starter = no good. My head is starting to hurt.

[CJH]

Yep (except it might take a while, depending on available sunlight and state of starter battery, before it climbs to the required 13.xx volts to kick in the smart relay)

Agreed, although in practice the starter battery is usually pretty well charged and I'd be happy for it to take priority anyway - I'd rather be without the leisure battery than the starter battery, and if necessary I could always bring out the jump lead.

Mind you, you say you have a zig panel, could you not wire the solar live to the "car / van" change over switch on that, then you could charge either, no, hang on, if you need to charge the starter battery (via solar, which would be rare occurrence when camping), then the switch would have to be set to "car" which in turn means your accessories would then be running off the starter = no good. My head is starting to hurt.

It's comforting to know that it's confusing you too

I've now settled on a situation where I never need to touch the ZIG switches (Mains charger always on, 'On-site/Touring' switch always set to On-Site so that the starter battery is isolated, and 12V supply always switched on). I could do without re-introducing a reason to fiddle with those switches thank you very much.

Actually though that's a good idea - you've reminded me that the 'touring' position links the batteries, which would allow me to charge both batteries from the solar, rather than having to wait for the smart relay to close, and without needing a jump lead. That position also disconnects the mains charger, but I'd only use it when there's no hook-up anyway.

So that's going to be easy to remember - when on site, set the 'On-site/Touring' switch to 'Touring'. Sigh....

I already carry a laminated schematic of the ZIG with me, with my own notes in the corner, just in case I can't remember how it works - it looks like I'll need to add to my notes.

[bigherb]

You probably won't have a problem in this case but charging two dissimilar batteries or differently charged batteries is a problem and can be dangerous.
It depends on the charger. If the charger never reaches the gassing voltage of a battery like an alternator then it will be fine.
If the charger can go over the gassing voltage older mains chargers and some smart chargers will to equalise cell voltages, then with dissimilar or unequal discharged batteries one battery will reach full charge before the other and will start to gas (hydrogen = dangerous) until the other battery reaches full charge.
Personally unless it is from a vehicle alternator I would never charge two batteries from a mains source better to put a change over switch in the circuit to select which battery you want to charge.

[CJH]

Ah, ok, so that might be a clue as to what happened with the battery in my photo above. What sort of voltage are we talking about for the gassing voltage? I've had a meter on my leisure battery when it's been charging from the ZIG, and I've not seen more than about 14.5V, usually it's under 14V I think. Similarly for the LIDL trickle charger. And I guess a solar charge regulator will be similar. I have a 14.5V regulator ready to go on my alternator, so I'm assuming the gassing voltage is higher than that.

[CJH]

Hmm - 14.5 is almost exactly the gassing voltage at 20 degrees C - see the table on this site - and it drops as the temperature rises. According to that page, the charging reaction takes precedence over the gassing reaction, so the voltage can safely be higher than this during the bulk charging phase, but this confirms that you need to be VERY careful charging two batteries with dissimilar charge states. A voltage regulator that kept the voltage below 14V, say, would keep it safe almost up to 40 degrees.

[ghost123uk]

Pretty much agree with what Herb says ^^^. This is why I mentioned that it should be OK as long as the Leisure battery is not foobarred.

Re that pic of those batteries, imho that wasn't caused by a slight excess voltage causing gassing, it was more likely they were either wired up wrongly or the charger was set to 24 Volts. Remember, this is "imho" of course, and I could be wrong

[CJH]

Re that pic of those batteries, imho that wasn't caused by a slight excess voltage causing gassing, it was more likely they were either wired up wrongly or the charger was set to 24 Volts. Remember, this is "imho" of course, and I could be wrong

Those batteries were simply wired in parallel (using Anderson connectors), and the 12V (only) charger was connected via another Anderson connector, so there was no chance of connecting them up incorrectly. I'm pretty sure the original problem was that one of the batteries was highly discharged and the other wasn't, and they were then connected to a high current charger and left to stew over a (long) weekend. We came in on the Tuesday morning to find the van flooded with steam and the batteries too hot to touch! With hindsight it's perhaps a good thing that the cases did allow a bit of the pressure to vent, otherwise we'd have had an explosion for sure.

[CJH]

OK, I didn't mean this to turn into a thread about my battery destroying exploits, but it's helping me to understand the dangers. The charger we used to destroy those batteries is a rebadged version of this one, which has an 'Equalizer' voltage of 14.6V. According to that earlier table, that's the gassing voltage at a little below 20 degrees C. So I'm pretty sure now that one of the batteries reached full charge before the other, so the apparent voltage of the battery pair, as seen by the charger, was less than the threshold at which it switches over from 'Equalizer' voltage (14.6V) to 'Floating' charge (13.7V), so it kept on charging at 14.6V. Since the charging reaction in the fully charged battery could no longer take precedence over the gassing reaction, it started to gas.

I suspect other things then led to the second battery failing too - if the first battery started to heat up, since they were strapped together the second one probably felt the heat. If the pair of them got up to 50 degrees say, that's close to the temperature where even the 13.7V float charge equals the gassing voltage.

[bigherb]

Hmm - 14.5 is almost exactly the gassing voltage at 20 degrees C - see the table on this site - and it drops as the temperature rises.

That's what I like to see someone using a bit of information and then doing some research well done.

Yes that is that is basically how it works. But it is a complicated process and important not to confuse two things

I keep trying to write something out but it's hard to condense a complicated process but as you can work things out I'll have a go.

To start a battery charging you need a minimum factor of 1 normally we use 1.2 to 1.4

So if we have a battery discharged to 12.2V then we need 13.2V to start it charging (some of you will notice that dispels a myth about battery to battery surges).

There are two ways to charge a battery, constant current variable voltage and constant voltage variable current we normally use constant voltage variable current on vehicles.
As you have found out voltage creates heat inside the battery if we use constant current variable voltage we can charge the battery at our factor of 1 at high currents till the battery is near full charged then it becomes a bit tricky.
So if we have a 70ah 12.2V discharged battery we can charge the battery at high amps something around the Ah capacity of the battery varying our voltage factor of 1 as the battery charges till the battery is about 80% charged then we have to reduce the amperage or switch over to constant voltage variable current. Constant current chargers are complicated kit and only really used on traction batteries in milk floats, forklifts etc.

Constant voltage variable current chargers are simpler and more automatic.
Our 12.2V battery initially will have a high voltage factor and will initially accept a high current but as the battery voltage rises the the voltage factor reduces until we reach near full charge the factor will reduce below 1 and can only push a small current into the battery. This is a safer and easier way to charge the battery but will take a longer time to charge the battery especially the last bit when the factor is below 1.

[CJH]

Thanks Herb, that's quite enlightening. Just one clarification for the moment, while I absorb the implications: the 'voltage factor' you are referring to is simply the difference between the charging voltage and the battery's voltage? That's how I've read it, and your explanation makes sense if that's the case, but I just want to be sure.

Actually - a second clarification if I may. The 'automatic' drop off in current in a constant-voltage-variable-current charger - is this a natural consequence of the voltage difference reducing, or is it actively controlled by the charger? So if I have a dumb charger (such as I think the ZIG charger might be) which simply stays at around 14V, will the current drop off naturally as the battery voltage rises? This would seem even safer than my lower current 'intelligent' trickle charger (when used to charge two batteries in parallel), since it's not going to try and charge at higher, potentially dangerous, voltages.

[dave friday]

Why don't you swap your "smart relay" for one triggered [sp] from the alternator circuit? then when the engine is off both charging circuits will be separate.

[CJH]

Why don't you swap your "smart relay" for one triggered [sp] from the alternator circuit? then when the engine is off both charging circuits will be separate.

Well yes, that's an option, but I quite like the way the smart relay works. Since it doesn't close immediately there's no danger of the leisure battery being connected into the circuit during starting, for instance. In fact, I've already got the smart relay set up to operate a second relay, so that the charging current isn't limited by the smart relay (one of Herb's earlier suggestions), so I could add an isolator switch to that relay I suppose. One more thing to forget though.

To be honest though, I'm still mulling my options while I learn more about the necessary precautions for battery charging.