Splitting aux battery into smaller gel cells

Submitted: Saturday, Nov 27, 2004 at 09:55

ThreadID: 18125 Views:3305 Replies:10 FollowUps:10

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Member - Chrispy (NSW)

Hi all :)

I'm hoping that someone might have some advice and experience in helping me with a battery problem I have. My new fourby is one of those ones that leaves enough room under the bonnet for a matchbox and a wire or two - to say the least it's tight for room.

I'm thinking of using my Piranha DB-150S isolator (the matchbox) to charge an auxilliary battery that I have to locate in the back of the cabin somewhere. Only problem is that I have 4WD Interiors' "Outback" drawer system installed, and the only room left is under its deck - down the sides of the drawer frame. This sliver of room is about 4 inches wide and a couple of feet long. I have access to good gel-cell 19AH batteries (free to me) that are similar to motorbike batteries in size - and four of them will fit lengthwise down this space. I'd then run thick cabling through the cabin and firewall to the isolator.

If I were to wire four of them up in parallel and call the 76AH array a "whole" battery, am I looking for problems? I suppose if any single battery were to die, I'd be none the wiser as to which failed, apart from feeling them to see if any are running hot (short).

This battery would only be used for running the fridge overnight (on low) and occasionally some flouro lights. No winch or any heavy-duty tasks involved.

Has anyone here on this forum ever done this before?

TIA
Chris

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Reply By: Member - Roachie (SA) - Saturday, Nov 27, 2004 at 10:24

Saturday, Nov 27, 2004 at 10:24

Chris,

I don't claim to be any sort of expert, but I can't see why your proposed system wouldn't work. The main concern would be the "weight" of the cable (must be heavy) and the fact that you have to come up into the body. The firewall might not necessarily be the best option.....perhaps you could consider running the cable along the chassis rail (encased in larger hose) and up through a rubber grommet in the floor etc.
In any case, the fundamental idea seems okay to me.....good luck

AnswerID: 86107

Follow Up By: Member - Chrispy (NSW) - Saturday, Nov 27, 2004 at 10:31

Saturday, Nov 27, 2004 at 10:31

Roachie - thanks :)

Yup - the cabling probably doesn't need to be all that thick - as it is only supplying enough current to actually keep the battery array charged. It won't be called on to supply a few hundred amps back to the winch for instance. I was thinking of using the figure-8 sectioned cable that is used for jumper wires - which is pretty thick anyway. I agree with you and think coming up though the floor is a better option than the firewall (grommet is already full).

Cheers
Chris

FollowupID: 344724

Reply By: Wok - Saturday, Nov 27, 2004 at 10:48

Saturday, Nov 27, 2004 at 10:48

Chrispy,

I did just that , but only 3 SLA from JAYCAR.....unfortunately I didn't reduce/control the charging current and the batts died prematurely. If you are getting true Gel cells you should be okay if your alt voltage doesn't exceed 14.1v.

rgds

ps: What $ can you get true Gel-cells for?...the 19Ah ones

AnswerID: 86110

Reply By: Member - Raymond - Saturday, Nov 27, 2004 at 11:09

Saturday, Nov 27, 2004 at 11:09

Hi Chris
As the price is right for the batteries, there is no problem running 4 or 5 in parallel and this will be fine for fridge and lights. I have justed wired a third battery into the GU and it is parrallel to the second battery. I have used Motorbike battery lead which is quite thick and this is going fine. The only problem is that it comes in black only, but some red insulation tape on each of the positive ends makes it easy, plus it is quite cheap to buy
Regards Ray

AnswerID: 86113

Follow Up By: GOB & denny vic member - Saturday, Nov 27, 2004 at 17:51

Saturday, Nov 27, 2004 at 17:51

goodday ray
rather than use tape why not get some heat shrink also helps stop fraying etc

steve

FollowupID: 344772

Reply By: Member - Chrispy (NSW) - Saturday, Nov 27, 2004 at 11:42

Saturday, Nov 27, 2004 at 11:42

Thanks fellas.... looks like the way to go.

I'm fortunate in that I have access to proper gell-cells for free. I use them at work in a stack of uninterruptable power supplies, and they only get a 4-month duty cycle and are then turfed for newies. This way I manage to put aside about 12 per year.

AnswerID: 86117

Reply By: hl - Saturday, Nov 27, 2004 at 12:52

Saturday, Nov 27, 2004 at 12:52

Hi,
I would be concerned to parallel connect them. As you already pointed out, if one cell goes short circuit you could end up seeing you pride and joy go up in smoke.
The other issue would be the charging current. There needs to be a current limiter, as most Gel cells can only handle a few amps initial charge. If you hit a flat gel cell with 14.1 volts, it may well try to take 15 or 20 amps, far too much for comfort and likely to produce the result mentioned earliera fairly soon thereafter.
So, you will need current limiting, as well as fuse protecting every parallel connection. Gets a bit messy!

Cheers
Horst

AnswerID: 86122

Follow Up By: Member - Chrispy (NSW) - Saturday, Nov 27, 2004 at 13:27

Saturday, Nov 27, 2004 at 13:27

Hmm... I see your point Horst.

Current-limiting isn't hard I presume, but as you say - sounds messy. I've heard of people using gel-cells in their cars as the second battery - primarily because orientation doesn't matter - you could lay them down on their side if need be with no ill effects. How do people who use these install them typically?

FollowupID: 344736

Reply By: hl - Saturday, Nov 27, 2004 at 14:27

Saturday, Nov 27, 2004 at 14:27

Hi again,

Well, there are of course some fairly big gel cells around that behave much like a car battery. I am talking about 60ah or more. They would require no special consideration except perhaps the charging voltage.
You will notice that Gel cells in cyclic use generally require a slightly higher charge voltage than a car alternator regulates to. This means that in normal use they would never get charged to full capacity which would make them expire fairly quickly.
So, there are a few hairs on it!
My preferred battery is the Exide Extreme. It is compatible with the main battery as far as charging voltage goes, is reasonably priced (especially when K-mart have a 25% off day!) so it does not cost the earth when it dies. In my experience most 2nd batteries die of heat exhaustion anyway, usually before the main because most main batteries have a cooling shroud.
But, that doesn't solve your problem, I guess. Back to the drawing board.
cheers
Horst

AnswerID: 86131

Follow Up By: Glenn D - Saturday, Nov 27, 2004 at 16:01

Saturday, Nov 27, 2004 at 16:01

Hows it going,
It's real easy to make a heat shield for your aux battery . My vehicle is a Patrol and the spot for the aux batt is next to the turbo, the whole area gets quite hot . A piece of sheet metal bent to wrap around the battery or some of that silver heat blanket work well, you can even use ducting to keep it cool. This would help extend battery life.
As mentioned earlier, its probably cheaper to kill a couple of Exide Extream's than one deep cycle battery!!

Glenn

FollowupID: 344753

Reply By: Richard - Saturday, Nov 27, 2004 at 19:13

Saturday, Nov 27, 2004 at 19:13

Had the same problem re second battery location. Bought a pirhana isolator and located a sealed battery in the third row seat well at the rear of the vehicle. Ran the supplied cable from the starter battery through the firewall to the isolator located under the passenger seat then to the battery in the rear. Cable runs under the carpet. Hard wired through fuses to the fridge, an inverter and two spare 16amp sockets. This has run well for about three years although one of the cells in the sealed battery died just out of warranty.

AnswerID: 86152

Reply By: Chaz - Saturday, Nov 27, 2004 at 19:15

Saturday, Nov 27, 2004 at 19:15

Hi Chrispy,
The idea you have will work, but the biggest problem with Gel Cell batteries is that they have no venting, so heat is criticle. If they get too hot they will warp and fail. Thats why they are no good under a bonnet, but if you can keep them cool enough they are great as deep cycle jobs. The other thing is to limit the charge rate from 13.2 - 13.5 volts and this will also limit the current. Definately have an isolator or better still a redarc solonoid and a 10 amp fuse to help limit the charge current.
The cable size will be determined by it's length, so you should get away with 4mm cable (OD). Also secure the batteries so they can't move around and make sure that nothing can short the terminals. A wooden or plastic cover would be great.
Having the batteries inside the cabin is a good idea because it's generally cooler and normally ventilated, and I would ground them to the floor pannel as close as possible to the batteries.

Good Luck
Chaz
http://members.bettanet.net.au/~conody/index.htm

AnswerID: 86153

Follow Up By: Member - Chrispy (NSW) - Sunday, Nov 28, 2004 at 08:19

Sunday, Nov 28, 2004 at 08:19

Thanks Chaz

I already have the spare Piranha unit - but I might investigate building a dedicated unit based on the redarc.... cheers :)

PS - Who's Grand Cherokee in your Gawler Rangers Cull March 2004 photo shoot?

Chris

FollowupID: 344820

Follow Up By: Chaz - Sunday, Nov 28, 2004 at 09:09

Sunday, Nov 28, 2004 at 09:09

No problem Chris,

The Cherokie belongs to a friend of mine (Malcolm) that's in our culling team. He's had Jeeps since I've known him and this one's a V8 petrol. I think he enjoys all the comforts of home when he goes camping and he's really well set up.

Chaz

FollowupID: 344827

Reply By: Member - Collyn R (WA) - Sunday, Nov 28, 2004 at 13:39

Sunday, Nov 28, 2004 at 13:39

I suspect hl is confusing gel cells with AGMs.

A gel cell will charge adequately from as little as 13.8 volts. Recommended is 14.1. They must never be allowed to go above 14.4 volts.

The concerns re shorted cells with parallel charging are massively exaggerated - and is in any case virtually non-existent with gel cells and AGMs.

You need not be concerned re heat build-up. Like AGMs but more so, gel cells have very low internal resistance and hence generate very little internal heat.

Note that the earlier battery makers' advice re ventilation has totally changed. Now all makers state that all batteries must be ventilated to atmosphere. This adviuce follows a number of US accidents where charging faults have caused batteries to be massively overcharged - to the point where the cases split and released large volumes of high pressure gas.
Trust this helps
Collyn Rivers

AnswerID: 86212

Follow Up By: Flash - Monday, Nov 29, 2004 at 11:33

Monday, Nov 29, 2004 at 11:33

Collyn's advice is, as usual, spot on.

I believe the setup you are contemplating will work just fine. DO check your output voltage from your alternator with an accurate meter to keep an eye on the whole system.
I would not use very heavy cable on the charge line (which will in itself help to reduce current) and DO put an appropriately sized re-settable circuit breaker in the circuit. That plus good wiring practice (Good wiring insulation etc) should give you a usefull aux battery system IMHO..
Cheers

FollowupID: 345014

Follow Up By: Member - Chrispy (NSW) - Monday, Nov 29, 2004 at 11:44

Monday, Nov 29, 2004 at 11:44

Thanks Flash

Is there any circuit diagram that anyone knows about so that I might be able to incorporate both current and voltage limiting within a single device?

Cheers
Chris

FollowupID: 345016

Follow Up By: Member - Chrispy (NSW) - Monday, Nov 29, 2004 at 11:48

Monday, Nov 29, 2004 at 11:48

PS - Collyn - what was your book called? I'd be very interested in a copy!

Cheers
Chris

FollowupID: 345017

Reply By: Member - Collyn R (WA) - Monday, Nov 29, 2004 at 13:39

Monday, Nov 29, 2004 at 13:39

Chris

The current (and next) issues of Silicon Chip magazine has exactly what you seek It is a three-stage constyant current/constant voltage charger with all the programability that could be needed.

I had some minor initial input re this (and suggested the project) - also had the prototype up here in Broome for checking.

Re my books. If you click on the 'Rig/Profile & Msg icon at the bottom of my posting you will find my website address. I have four books on sale - and am working on a fifth (specific to the 4WD/camper trailer field) right now.

There is a swag of free technical info on the site - and no hard selling!.
Collyn Rivers

AnswerID: 86390