Adding a second leisure battery

[CJH]

Here's a thread to stop Ghost getting bored maybe.

I'm after advice on how to wire in a second leisure battery. I thought my starter battery was on its way out so I bought a new one, but it turned out I had a fault in my electrics that was draining it, and in fact my starter battery is fine - still holding over 13.1V nearly 12 hours after charging it. So now I've got three good batteries, and I want to think about making use of all of them.

My existing leisure battery is an 80AH leisure battery from Alpha - I bought it in July this year. It's wired via a Smartcom relay to get a charge from the alternator, and via a ZIG CF8 to get a charge from a mains hook up. All the load wires are connected through an LVD (the 16A one that Ghost recommended), and I leave the ZIG switches set so that the starter and leisure batteries are disconnected from each other at all times.

I'm wondering what's the best way to wire my spare battery into the circuit. If I just add it in parallel to the existing leisure battery, there are a few problems that I can see:
1) The current through the Smartcom is more likely to exceed its limit. However, one nice thing about the smartcom is it doesn't switch in until the voltage rises to a given level, so this circuit never sees the high currents experienced during starting.
2) The LVD will only see the combined battery bank, and won't know the state of charge of the individual batteries. Potentially one battery could drop below the ideal cut-off voltage, even though the bank is above the LVD cut-off point.
3) Mains charging through the ZIG will be in parallel (as would the alternator charging in fact). It's a 10A charger, which I know isn't particularly high, but I'm very nervous of charging batteries in parallel, due to the potential for them to be at different states of charge. If one battery is 'full' and the other isn't, then an intelligent or phased charger might continue to charge at a higher current than the full one needs. I have first-hand experience of this in another vehicle!



This seems all the more likely because the batteries aren't matched - one is an 80AH leisure battery and the other would be a 72AH starter battery.
4) I don't know how the parallel bank will distribute its power - will one battery drain faster than the other, due to their different characteristics?

So what I'm wondering is whether there's a more sophisticated way to do this. I can solve the Smartcom current issue by using its fridge output to trigger a separate dumb relay. But parallel 'dumb' charging and low voltage protection would still be a concern. I'm also hoping to fit a 100W flexible solar panel at some point. So I'm wondering if a dual, or even triple, charge controller will solve all my problems. Does a charge controller monitor each battery separately so that the charge for each battery is administered intelligently, and so that the load is balanced between them? Can I use a charge controller to manage all three sources of charge (alternator, ZIG, solar) across all three batteries?

[ghost123uk]

The common wisdom is not to wire dissimilar batteries in parallel, but...

I do it

I will explain. I fly model aircraft and use a motorcycle sized battery as a model engine starter battery. It is often found sitting next to my 110Ah leisure battery, wired in parallel (via plastic covered croc clips on the small battery) via fairly thick wire (20A at a guess) with a 15A fuse in line. I have never had a problem with this mismatched set up. Note = I do disconnect the smaller battery when using the vans leisure loads when camping.

Now, if one of your 2 batteries "settles" at a much lower voltage than the other (when fully charged, rested, not connected to anything) then the better one (higher resting voltage) will feed into the other until (very quickly) both will be at the same (lower) voltage. This just means that the "higher" voltage one will have used maybe 5% of it's capacity by dropping to the lower voltage = no big deal.

When you wire them in parallel they in effect become one battery so there is no danger of overcharging just one.
They will run down exactly the same as each other, the voltage will drop the same on both because they are wired together in parallel. The charging current will be higher of course, but note that when charging via the alternator, into a battery, (or 2 batteries in parallel) high currents only flow for a minute or two, rapidly reducing to a lower value as the initial low voltage of the batteries has risen.

This is due to "potential difference" ie, if the battery(s) is at 11 volts and the alternator is chucking out 14 volts, there is a PD of 3 volts and a hefty current will flow. But as the battery(s) receive this high current (say 30 amps) they VERY quickly start to have a higher voltage, so the PD is less and less current flows. By the time the batteries are charged they "sit" at about 12.9 volts, then the PD is only 1.1 volts so only a trickle of current flows into them. This is how "dumb" domestic chargers work. You can prove all this by measuring the voltage rise across the battery(s) with a meter on firing the motor up with the battery(s) in a depleted state (like after the LVD has cut them off)

In conclusion then, it is pretty much the case that you simply add the AH rating of the two batteries together and look at them as one big battery (ie in your case 152AH)

What amp rating is the Smartcom ?
Put an in line fuse in the output wire that is a tad lower than the relay rating. If it does not blow, all is good to go.

I am not sure however that I would connect 2 batteries so dissimilar to use as a leisure system. Can't put my finger on why, but it just doesn't seem right. I might however buy a changeover switch and wire it so that if my main leisure battery was cut off by the LVD when camping, I could flick a switch that would operate a relay to in effect "swap" over to the other battery. Yes, that is what I would do.

~~~~~~~~~~~~~~~~~~~~

Jeez, how did you get that battery in that state ?
I would say you were VERY lucky it did not explode!
I have seen a smaller battery explode (like a motorbike battery). it was in a model boat and the guy connected it the wrong way round to his car via jump leads to charge it. It went off like a bluddy bomb. Luckily the guy had walked a few yards away before it went off. Not funny at all

Take care and use fuses

[bigherb]

If you need more charging capacity then make sure the cable is minimum of 4mm, 6mm is ideal between the batteries then use one of the outputs of the Smartcom to switch a normal 70A relay and leave the other output to just switch the fridge if it is wired to it.

[AdrianC]

The common wisdom is not to wire dissimilar batteries in parallel, but...

...unless you've got a Sterling or similar clever alternator charge manager, it's exactly what happens when the split-charge relay kicks in and the alternator starts to charge both leisure and starter battery.

You won't see higher currents passing, because the charging and the load are exactly the same as now - you'll just have more of a reserve.

As for that battery - you're not meant to use a tyre pump on it...

[CJH]

Jeez, how did you get that battery in that state ?
I would say you were VERY lucky it did not explode!
I have seen a smaller battery explode (like a motorbike battery). it was in a model boat and the guy connected it the wrong way round to his car via jump leads to charge it. It went off like a bluddy bomb. Luckily the guy had walked a few yards away before it went off. Not funny at all

Take care and use fuses

That swollen battery was one that was in a vehicle we use at work. We used to have what we thought was a nice flexible solution for providing power for equipment in the vehicle. We had a number of 110AH batteries with those big interconnects attached so that we could make up a bank of one, two, three or more depending how much power we planned to use. We installed a big intelligent charger (30-40A, can't remember for sure), and had it such that we could plug the vehicle into the mains overnight and charge the battery bank in parallel. That charger was big enough that we could guarantee having all the batteries fully charged by the morning, no matter how much they'd been used the previous day. It worked fine for a while.

Then one morning we discovered the van was full of white steam, the batteries were hot and swollen (the one in the picture was one of a pair in use at the time, and the other was in a similar condition). We immediately disconnected the mains hook up, and rolled the van out of the garage and opened all the doors. We're well aware of how lucky we'd been - very close to an explosion, which would probably have led to a fire (still connected to the high current charger), which would probably have taken the rest of our office building with it! So naturally we don't do it that way any more! All the batteries have to come out to a ventilated charging station, and have separate chargers on them.

I'm still not sure what caused it, but I have a theory. The charger is fine, and still operates a two-phase charging cycle correctly, i.e. it drops to trickle charging when the battery is full. What I think must have happened is what I described in the first post. I think the batteries were at different charge states and the charger kept pumping 30-40A into the combined battery bank, even though one was full. I guess this started to boil the full battery. I'm not sure why the second battery would have done the same thing though. Could it be that the first battery buckled a plate, caused an internal short, and effectively reduced the battery's voltage? This would mean that the charger would never sense that the bank was fully charged and would just keep on giving its maximum current. Is that what happens when battery cells short? Is a 12V battery made from a series of lower voltage cells, such that shorting a couple of cells would give a lower voltage?

I am not sure however that I would connect 2 batteries so dissimilar to use as a leisure system. Can't put my finger on why, but it just doesn't seem right. I might however buy a changeover switch and wire it so that if my main leisure battery was cut off by the LVD when camping, I could flick a switch that would operate a relay to in effect "swap" over to the other battery. Yes, that is what I would do.

I'd considered a simple switchover, and yes it seems like a more foolproof solution. I think I'd probably not even bother with the relay, maybe just use a high current switchover toggle or rotary switch, and place it in such a way that the LVD (and hence all the loads) switches over. But I'm a bit disappointed that there isn't an all-purpose intelligent charge/load controller. Something that would allow me to connect multiple batteries to separate pairs of terminals, and multiple power sources (with at least some terminals having the intelligence needed to maximise the power available from a solar panel). Ideally it would distribute charge to each battery separately, so it could monitor the voltage of each battery independently and stop charging any that are full. Ideally it would also allow me to specify which batteries would be used for the 12V load, so that I could specify one of the batteries as the starter battery such that it would receive charge but wouldn't be used for the leisure circuits.

[CJH]

If you need more charging capacity then make sure the cable is minimum of 4mm, 6mm is ideal between the batteries then use one of the outputs of the Smartcom to switch a normal 70A relay and leave the other output to just switch the fridge if it is wired to it.

Ah yes, of course, so none of the charging current goes through the Smartcom - it's just a clever switch. Makes sense.

...unless you've got a Sterling or similar clever alternator charge manager, it's exactly what happens when the split-charge relay kicks in and the alternator starts to charge both leisure and starter battery.

Yes, the fact that both batteries are connected to the alternator hadn't escaped me, and I can't quite reconcile the advice that different types of battery shouldn't be charged in parallel (and my own near-catastrophic experience) with the fact that thousands of people use split charge relays with mismatched batteries all the time.

That Sterling charge manager sounds interesting - do they make one that'll do what I described above?

[AdrianC]

...unless you've got a Sterling or similar clever alternator charge manager, it's exactly what happens when the split-charge relay kicks in and the alternator starts to charge both leisure and starter battery.

Yes, the fact that both batteries are connected to the alternator hadn't escaped me, and I can't quite reconcile the advice that different types of battery shouldn't be charged in parallel (and my own near-catastrophic experience) with the fact that thousands of people use split charge relays with mismatched batteries all the time.

That Sterling charge manager sounds interesting - do they make one that'll do what I described above?

Lloydy's your Sterling expert. At the very outside, though, you could just use one Sterling per battery...

...but...

http://www.sterling-power.com/products-altbatt.htm" onclick="window.open(this.href);return false;


They show a bunch of daisy-chained leisure batteries...

[CJH]

[
Lloydy's your Sterling expert. At the very outside, though, you could just use one Sterling per battery...

...but...

http://www.sterling-power.com/products-altbatt.htm" onclick="window.open(this.href);return false;


They show a bunch of daisy-chained leisure batteries...

Thanks Adrian - so they do. I'll have a good look around their website.