Adding a third battery?

Submitted: Sunday, May 01, 2011 at 13:15
ThreadID: 85991 Views:4606 Replies:6 FollowUps:5
This Thread has been Archived
Hi all,

To the learned ones amongst you all, I ask this question.

I currently have 2 Trojan 120 ah in my van wired in parallel (Batteries 15 months old). I have 220 watts of solar panels on the roof. I also have the car wired to the batteries from my auxiliary battery, to charge them while I'm travelling.
My question is, can I add a third battery to give me greater storage capacity, (I have heard that you can't add a new battery to older ones, even though they aren't that old?)
I haven't room to add more solar panels so was hoping they would be fully charged from the car and then the solar panels would help when the sun shines.
I have a Dingo 12 48 volt 20 amp regulator. And finally if I can add another battery (Same type and size) how do you wire a third battery in parallel.

Kind regards from Col.
Back Expand Un-Read 0 Moderator

Reply By: pepper2 - Sunday, May 01, 2011 at 15:54

Sunday, May 01, 2011 at 15:54
yes you can have third battery in parrallel wire it positive to positive and negative same as first two ie connect 3rd battery to second battery as if you were jump starting 2nd batt with 3rd batt.

It is best to have all three batteries the same type and age if possible but not absolutely necessary,note if one of the three batteries has a dead cell it will discharge all three if it is not isolated.
AnswerID: 452927

Reply By: Member Al (Sunshine Coast) - Sunday, May 01, 2011 at 16:05

Sunday, May 01, 2011 at 16:05
Hi Col, Yes you can safely add a third battery in parallel. Best that it is the same type and size. Adding a new battery to the older ones is not a problem. A small problem arises when eventually one of the batteries begins to fail. If it begins to self-discharge due to displaced plate material then it will also discharge the connected batteries. If this begins to happen you will observe depleted capacity from the bank and it is simply a matter of separating the batteries to test them individually. Then renew the faulty battery.

To connect the third battery in parallel it is simply connecting its positive to the others positive and negative to negative. Provided that the interconnecting cable is very short and carefully positioned to avoid damage there is no need for fuses in the interconnecting cable. However if the third battery is at all remote from the others then it is necessary to place fuses in the positive cable only. Two fuses, one close to each battery positive terminal. 30 Amp fuses should be OK.

Cheers
Allan

Member
My Profile  My Blog  Send Message

AnswerID: 452933

Reply By: Battery Value Pty Ltd - Sunday, May 01, 2011 at 16:34

Sunday, May 01, 2011 at 16:34
Hello Col,

as has been said, no problem in wiring a new battery in parallel to the two older ones.
You can try to fuse the three batteries lightly, so that if one battery drops a cell, it isolates itself by blowing its fuse.
Start with 3x 15A, and if they blow (which I doubt they will) due to high alternator charging current, replace them with 20A ones.

If you want to keep an eye on the battery current distribution, you can wire a voltmeter between the batteries. As soon as a cell in one of the batteries starts to go down, one of the two voltmeters (or both, if it's the one battery in the middle) will start reading several tens or hundreds of milliVolts.
See the image for better illustration below - just add the 3rd battery with its own fuse, and another voltmeter if you prefer.

Image Could Not Be Found

cheers, Peter
AnswerID: 452940

Reply By: Member - colin J (VIC) - Sunday, May 01, 2011 at 17:42

Sunday, May 01, 2011 at 17:42
Hi guy's

Thanks for that info, very much appreciated. Will the 220 watt of solar panels struggle to charge the 3 batteries or will it still be of some benefit to have the third battery with the set up I have?

Thanks from Col.
AnswerID: 452944

Follow Up By: Member - John and Val - Sunday, May 01, 2011 at 18:05

Sunday, May 01, 2011 at 18:05
Col,

The panels won't struggle any more with the extra battery than they do now. The extra battery will provide more storage, not higher demand. To get full benefit from that storage you may need additional charging capacity, either more solar or more from the vehicle. Depending on your usage, and given favourable sun conditions, I'd expect your 220W of solar capacity to be adequate. I think I'd be adding a dc-dc charger to beef up the charge coming from the vehicle, rather than adding more solar. (Especially if you are short of solar space.)

From the specs I'm not sure that your controller is an mppt type. If it isn't, you could probably get a 20% solar increase by installing an mppt controller.

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

Lifetime Member
My Profile  My Blog  Send Message

0
FollowupID: 725698

Follow Up By: Battery Value Pty Ltd - Sunday, May 01, 2011 at 18:21

Sunday, May 01, 2011 at 18:21
You're welcome Col,

but your last question can only be answered if the daily load requirement is known.
Your solar panels can produce about 60Ah per day (average figure).
Just assuming your daily load requirement is about the same figure, then you can ride out two or three overcast days without starting the motor.
Add another 100Ah battery for each additional overcast day you don't want to start the motor.
Keep an eye on the battery voltage monitor.
Voltage should reach 14.4V by 2pm, and stay on 14.4 until 4pm or until the sun sets.
If this absorption voltage of 14.4V can't be reached by 2pm on a regular basis by solar alone, start the motor in the morning.
Increase the daily run times in 15 minutes increments until you hit the time/voltage window as described above.
As a rough guide, one minute of motor run time equals one Ah charge to the batteries.

cheers, Peter
0
FollowupID: 725699

Follow Up By: Member Al (Sunshine Coast) - Sunday, May 01, 2011 at 21:00

Sunday, May 01, 2011 at 21:00
Col, as John says above, the extra battery will provide more storage, not higher demand. You can think of the batteries as three identical water tanks connected together. You now have more storage capacity..... more water (electricity) to use. But you will have to put back as much as you have used to fill the tanks up again. This of course will take longer than if you had used less. But both the alternator and the solar panel will charge at the same rate as with two batteries, simply taking longer to "fill up" the increased capacity.

Cheers
Allan

Member
My Profile  My Blog  Send Message

0
FollowupID: 725732

Reply By: Member - John and Val - Sunday, May 01, 2011 at 17:51

Sunday, May 01, 2011 at 17:51
Col,

I've nothing to add to the earlier replies except to suggest that if you intend using the vehicle for most of your charging, a dc-dc charger near the batteries might be a better option than additional solar capacity. These chargers take the available voltage sent back to the van from the alternator, and increase it so as to increase the charging current going to the batteries. This can overcome losses in wiring and ensures that you batteries can in fact be fully charged by the vehicle. One side effect is that you may need heavier cable running from the engine bay to the van (suggest at least 6 B&S twin), but that may be the case anyway as you'll have more storage to charge.

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

Lifetime Member
My Profile  My Blog  Send Message

AnswerID: 452946

Follow Up By: Battery Value Pty Ltd - Sunday, May 01, 2011 at 18:53

Sunday, May 01, 2011 at 18:53
Hi John,

Generally, DC/DC chargers are better than pure alternator charging, but are inferior compared to well tuned solar setups.

And the OP already has a top specced, highly configurable solar regulator.
Not a single DC/DC charger comes close to the Dingo in this regards.

The OP only needs to supplement the daily solar output with some alternator charging sessions in the morning. And for this he wants max alternator charging current, which the DC/DC can't deliver, while the batteries' SOC is low.

I hope this explains, why I don't suggest a DC/DC converter in this case.

cheers, Peter
0
FollowupID: 725706

Follow Up By: Member - John and Val - Monday, May 02, 2011 at 08:42

Monday, May 02, 2011 at 08:42
Hi Peter,

Good point, though very dependent on the standard of vehicle wiring. With 3 batteries in parallel there is capacity to easily absorb 50+ amps. The constraint will be the voltage actually reaching the batteries, allowing for wiring losses at high current. While I fully accept your reasoning, in many cases I expect that charge current will be restricted by wiring limitations to a point where a dc-dc charger would make a valuable contribution. I see that Col has said that the wiring is good, so in this case, I bow to your wisdom!

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

Lifetime Member
My Profile  My Blog  Send Message

0
FollowupID: 725755

Reply By: Member - colin J (VIC) - Sunday, May 01, 2011 at 18:46

Sunday, May 01, 2011 at 18:46
Hi all,

Thanks again for the additional info. I have no more space on the van's roof but do have the correct wiring from the car and will take on board all of your suggestions. We camp nearly always remote and after 2 or 3 days of overcast conditions I have to start the car which is no hassle really but another battery will give me peace of mind. But being from Vic the sun always shines after a day or two anyway Ha!
I'm one happy chappy for with this info I'll be able to put another battery beside the others and the better half won't have to be told to go easy on the power usage and I won't have to cop the earful of why things aren't coping.

Thanks again for your replies,

Col.
AnswerID: 452951

Sponsored Links