Triple battery system question?

I am putting together a 3 battery system and I have come across what maybe a problem.
The system is a redarc 1225lv charger, an 80 ah battery under the bonnet, and a 120ah removable 'battery in a box'. The removable battery is for running the campsite,( fridge, freezer maybe, lights,battery charging) and will have a 150 w solar panel array to recharge during the day, topped up with a genny/smart charger if necessary. The 80ah stays in the ute to run a fridge when away from the camp detecting, charged by the redarc, and a solar panel when stopped, probably 80/100w.
When leaving home, both batteries will be fully charged, and whilst camping will be seperate as above. When I leave for home, or move to another site the idea is they both connect to the redarc, but I am concerned that, as they are different sizes, and at that point likely different states of discharge, may I be risking over charging one or the other?
Is there some form of 'over charge protection' I may fit to each battery to prevent this?

Thanks

As far as the battery in the box goes I think it would be ok to be charged separately. You make no mention of the 3rd battery size. The 2nd and 3rd battery should be of the same age and capacity to be on the safe side.

Have used two of these solenoids to run a simple system for years without any problems. http://www.ebay.com.au/itm/COLE-HER...7cfa81&pid=100338&rk=2&rkt=18&sd=252370340299

Starting battery (1) & auxiliary (2) separated with first solenoid connected to accessories on ignition. (No drain on battery 1 while stationary)
Third battery (3) under tray separated by second solenoid from batteries 1&2 with individual on/off switch.
Battery 1 & 2 charged by alternator while driving.
Battery 3 can be charged via on/off switch while driving, 64W roof mounted solar and 120W portable fold up panel when stationary.
Battery 3 can also be charged with 40amp 240V/12V battery charger connected to generator on cloudy days.
Battery 3 runs two Engels one on fridge the other on freeze + accessories + charge detector & radio batteries.

Yes, those solenoids are good. I installed a dual battery system using them years ago in my brothers Series III LandRover and it's still going strong. A useful "peace of mind" addition was a simple relay circuit connected to green and amber LED pilot lamps mounted in the cab that give a visual indication of the solenoid state :-

GREEN = engine running and AUX BATT connected

AMBER = engine stopped and AUX BATT disconnected (if engine is running AMBER means that there is a circuit fault)

casper

cairnspom said:

I am putting together a 3 battery system and I have come across what maybe a problem.
The system is a redarc 1225lv charger, an 80 ah battery under the bonnet, and a 120ah removable 'battery in a box'. The removable battery is for running the campsite,( fridge, freezer maybe, lights,battery charging) and will have a 150 w solar panel array to recharge during the day, topped up with a genny/smart charger if necessary. The 80ah stays in the ute to run a fridge when away from the camp detecting, charged by the redarc, and a solar panel when stopped, probably 80/100w.
When leaving home, both batteries will be fully charged, and whilst camping will be seperate as above. When I leave for home, or move to another site the idea is they both connect to the redarc, but I am concerned that, as they are different sizes, and at that point likely different states of discharge, may I be risking over charging one or the other?
Is there some form of 'over charge protection' I may fit to each battery to prevent this?

Thanks

Click to expand...

...... it's not that critical if the batteries are different sizes because when they are connected together they are a single load not two in other words they become effectively a single battery. Your vehicle alternator or your chosen battery management device will see them as a single load to charge and there will be no problems.

casper

In a nutshell parallel charging allows for multiple same cell count batteries to be connected in parallel to create a single larger logical battery that can then be charged.
http://www.tjinguytech.com/charging-how-tos/parallel-charging

Jaros said:

As far as the battery in the box goes I think it would be ok to be charged separately. You make no mention of the 3rd battery size. The 2nd and 3rd battery should be of the same age and capacity to be on the safe side.

Click to expand...

Sorry should have been clearer, the 3rd battery is the engine cranking battery, unfortunately I can only fit a 80ah battery as the 'first' auxiliary under the bonnet due to size constraints, but need the capacity of the 120ah to run the camp. that is why I am worried about charging batteries of different sizes, in different states of charge. The 80 could be fully charged while the 120 may be down 25%, and with them both connected to the dc/dc charger I am worried I may risk overcharging the 80

cairnspom said:

Jaros said:

As far as the battery in the box goes I think it would be ok to be charged separately. You make no mention of the 3rd battery size. The 2nd and 3rd battery should be of the same age and capacity to be on the safe side.

Click to expand...

Sorry should have been clearer, the 3rd battery is the engine cranking battery, unfortunately I can only fit a 80ah battery as the 'first' auxiliary under the bonnet due to size constraints, but need the capacity of the 120ah to run the camp. that is why I am worried about charging batteries of different sizes, in different states of charge. The 80 could be fully charged while the 120 may be down 25%, and with them both connected to the dc/dc charger I am worried I may risk overcharging the 80

Click to expand...

To recap earlier posts .... two small batteries connected in parallel actually become one large battery. Your dc/dc charger will see ONE BIG BATTERY and if it's working okay it will not overcharge them.

casper

I think Condor may drop in and elaborate, but to my knowledge....

When a battery is low on charge, its resistance is lower, and it requires more current to recharge...
When paralleling 2 batteries, with a massive charging capability attached, the batteries will basically take what they want and then protest...
I dont think that one battery will be over charged, whilst the other battery is being charged..
But I am willing to be learned on this matter..

Yup in the GT digging, but here's my take.

Yes the charger will see one big battery, BUT - Here's a scenario for you, re an 80AH and a 120AH joined to be charged in parallel.

Assume the 80AH is at 95% charged i.e only 4AH used. It's charging requirement would be in the last stages of Absorption mode i.e. bugger all Amps
then,
Assume the 120AH is at 50% capacity so 60AH used it's requirement is in Bulk Charge mode, i.e. pumping in high Amps.

A smart charger sees the overall, decides the bank needs Bulk mode and you are thumping the hell out of the 80AH which is close to only wanting Float mode.
(the figures are, as above - total 200AH of which 65AH used = 32.5% used = BULK charge till about 90% charged)

My opinion, depending on the difference in state of charge of either battery, if definitively known, it's not a place I personally would want to stand too close to.

FACT

• You can charge multiple batteries of the same age type and capacity if they are in parallel when discharged and remain so when charged.[/*]
• I would definitely NOT recommend charging multiple batteries that are discharged separately or of different capacity. a. It could be dangerous, b. The batteries will suffer. [/*]

Charging disparate capacity batteries using smart chargers will cook the smaller battery so to use dc-dc chargers you'd have to get another charger for the 120a battery or get a multi output dc-dc charger. If your alternator is the normal regulated type you could get away with a FET isolator which would be a much cheaper option and charge the 120a battery in parallel with the starter battery off the alternator. The isolator will prevent one battery equalising with the other.
Jon

Hmm thanks guys, I was afraid that might be the case. I think I will have to put a switch in line with each battery, so when leaving camp (the only time they are likely to be in states of different discharge) I can charge each battery singly, until they are both definitely in absorption, or even better, float mode before putting them back together in parallel, that is if I am correct, and once they are in float, the difference in size wont matter

Your charger is rated for 75-200ah capacity so with the 80 plus the 120 you are on the design limits at 200ah. I would assume the float should be on the same output circuit as other modes and should be able to provide the required current to both batteries and stay in float but the pamphlet for the charger says:

"The BCDC charger then switches to the float
stage where it retains the 100% charge until a
load on the auxiliary battery causes the battery
voltage to drop below a predetermined voltage
where it then re-enters the boost stage."

https://www.redarc.com.au/Content/Images/uploaded/Flyers/Redarc brochure - BCDC Range.pdf

This information suggests to me that if you have a fridge running then the float mode might not supply enough current and the charger will return to the boost mode and you will have overcharge problems on the smaller battery. There is no harm in testing your float idea though. If it doesn't work or sounds a bit risky I would get a 10a charger for the 80ah battery or use an isolator as mentioned above. One thing though when the battery drops to float it is charged so there's no reason to keep it on charge if it is not being used so no need to put them back in parallel.
Jon

Thanks Jon, I think the isolator on the 80ah is probably the way to go, generally, the way I envisage their usage it will never really be discharged, its main use is to keep a fridge running daytime when out detecting with the ute, and it will have a small solar panel topping it up (small engel, should only need 2.5 A/h max) so the 120 is the one that will do all the work!As there isn't a need to re-connect them (in parallel) I will look for a switch to go from the redarc to either or so mistakes cant be made!

Nigel

cairnspom said:

Hmm thanks guys, I was afraid that might be the case. I think I will have to put a switch in line with each battery, so when leaving camp (the only time they are likely to be in states of different discharge) I can charge each battery singly, until they are both definitely in absorption, or even better, float mode before putting them back together in parallel, that is if I am correct, and once they are in float, the difference in size wont matter

Click to expand...

The batteries are different, the amperage is different, the age is also probably different - The industry recommendation is DO NOT PARALLEL at all. In other words only use a switch to charge one or the other, but to float them both you will need 2 chargers. Ask yourself this, they are different, which one will the charger use to sense condition?

I have a 100AH in the 4x4, I also have a 120AH in the caravan. I have a Redarc next to the 100AH in the back of the 4by. I also have another Redarc next to the caravan 120AH, supplied via Anderson plug from the 4x4.

The end result of saving money on a 2nd charger, will be the premature replacement of, most probably, both batteries.

Thanks Condor, yes, I think I understand now. In fact both batteries are brand new, but as you have stated that makes no difference. My plan now will be to put a 3 position isolator on the output side of the charger, off, battery 1, or battery 2 so they will never be paralleled. I dont expect the 80ah to ever really need charging from the redarc in normal usage, as it will only really be connected to a fridge in daytime, with solar charging as well (and the engel is only rated at 2.5 A) although it means it is available for other uses when not camping, and just using the ute. The 120ah will be charged from solar when camping (and genny/charger if necessary) so its only when moving camp or going home it will need charging from the redarc.

Nigel

As Condor said sometimes you're better off with a correct solution. The charger should be protected but going from float to no load to boost might not be a good thing on the output. If you use a battery isolation switch make sure it is a make before break switch so there is no disruption to the output of the charger. This will very briefly put the batteries in parallel as you switch. You can also get make before break relays which will switch much faster approx. 10ms.
Jon

Ok Jon, I thought I had a handle on this, and you've just confused me again! I think what you have said will not matter as it will only be switched when the ute is not going, but can I ask why this might be necessary?

Nigel

cairnspom said:

Ok Jon, I thought I had a handle on this, and you've just confused me again! I think what you have said will not matter as it will only be switched when the ute is not going, but can I ask why this might be necessary?

Nigel

Click to expand...

Normally switch/relays will break the existing battery connection before connecting the other battery so there is a momentary loss of connectivity to both batteries in this case. I am not sure how the redarc would react in that situation if it is charging (it might actually handle it ok). If the ignition is off then it won't matter. Look for a SPDT relay for this purpose with the charger positive wire to the pole and the normally closed relay tab to your 80a battery positive and the normally open tab to your 120a battery positive. Bolt the charger ground wire to the chassis. Flick a nice led illuminated switch to engage the relay to disconnect the 80a and charge your 120a. The relay coil and switch should be wired from your ignition.

Jon

The kind of switch you might need is a 3 position, centre off. In that way you go from one battery to off, then to the 2nd battery. Give a few seconds pause whilst in the off position.

Simple and no need for relays. Just make sure the switch will handle the charger current.

The type is a DPDT = Double Pole Double Throw