What size solar panels to charge 2 batteries?

Submitted: Monday, Sep 23, 2013 at 10:04
ThreadID: 104443 Views:3250 Replies:9 FollowUps:4
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I have a 97Ah dual battery under bonnet and 120Ah in camper. What size (w) do I need to charge these?
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Reply By: Ozhumvee - Monday, Sep 23, 2013 at 11:18

Monday, Sep 23, 2013 at 11:18
Really depends on your usage, but panel capacity should be greater then the battery capacity ideally. If trying to keep up with a compressor fridge and other high drain appliances then more is better.
So 217 a/hrs of battery will probably require 220 watts of solar but if you can fit more do so plus a good solar regulator as close to the batteries as possible.
If going portable then a 120 watt for the aux and 150-200 for the camper.
AnswerID: 518631

Reply By: John and Regina M - Monday, Sep 23, 2013 at 13:06

Monday, Sep 23, 2013 at 13:06
Any panel greater than about 10 watts will charge both of them.
But if you want them charged quickly, that's a different equation.

Relevant information would assist.
AnswerID: 518633

Reply By: Dennis Ellery - Monday, Sep 23, 2013 at 13:34

Monday, Sep 23, 2013 at 13:34
Depends on your useage
I have a 40l Engel run as fridge and a 37l Waeco run as a freezer and the wife runs a TV a couple hours a day with lead lighting in the caravan.
We have 280Watts of panel and still need to run the genset to top up batteries.
I have a fast charge system where I put a 100 amp charge into a 150 amp hour battery - so I only run the genset for half an hour a day.
AnswerID: 518634

Follow Up By: Member - Ups and Downs - Wednesday, Sep 25, 2013 at 08:43

Wednesday, Sep 25, 2013 at 08:43

I hear that those 'lead' lights are heavy on power.

FollowupID: 798615

Follow Up By: Dennis Ellery - Wednesday, Sep 25, 2013 at 11:06

Wednesday, Sep 25, 2013 at 11:06
Yep I knew I was in trouble as soon as I hit the submit button.
FollowupID: 798620

Reply By: Garry S3 - Monday, Sep 23, 2013 at 17:21

Monday, Sep 23, 2013 at 17:21
It really depend on what you want to run and how quick you need those batteries charge up. very often, 120w portable panel is good enough to run a small camping fridge and some lights. Again, depending on whether you are planning to stop by for a long period of time or 2 days at most. If you want to be 100% sustainable, I reckon you will need 220w at the very least. or another safe way to go about it is to have a generator for back up. always have a back up system, you never know what might happen out there. good luck
AnswerID: 518650

Reply By: Twinkles - Monday, Sep 23, 2013 at 17:28

Monday, Sep 23, 2013 at 17:28
Thanks everyone. Have an old 29 or 39 Engel. Camper has 3 way fridge. LED lights.
AnswerID: 518651

Follow Up By: Member - Bruce C (NSW) - Monday, Sep 23, 2013 at 18:27

Monday, Sep 23, 2013 at 18:27
Hi Twinkles,
The rule of thumb is 100 watts for every 100 amp hours of battery you wish to charge.
As the system has more appliances applied to it and drawing more from the system then you need to increase the amount of solar panels and then the batteries depending on your requirements.

Naturally if you increase the solar panels then you need to increase the size of your regulator as well or run a couple of regulators on a few panels each. Requires some thought to get it right if increasing beyond the 30 amps of charge point.

Cheers, Bruce
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restless and lost on a track that I know. HL.

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FollowupID: 798568

Reply By: Mike S2 - Wednesday, Sep 25, 2013 at 09:23

Wednesday, Sep 25, 2013 at 09:23
My system is 2x180w panels thru 30amp mppt solar charger to 2x120ah agm batteries seems to be more than adequate on good sun days but have had to genset top up at other times mostly our wintery cloudy hardly see the sun days,regs,Mike
AnswerID: 518711

Reply By: Member - John and Val - Wednesday, Sep 25, 2013 at 09:47

Wednesday, Sep 25, 2013 at 09:47
Hi Twinkles,

As already said, depends on your usage, because basically,you must put in what you take out, plus 10-20% to make up for inefficiencies. Having one battery at the front, and one at the back confuses things a bit, since they will have different loading and so different charging needs. Because of the distance between them, the two batteries need to be considered separately, even if they are electrically connected together. The Engel will be the big consumer (assuming that you don't run the 3 way fridge from a battery - that is important, since a 3way fridge running on 12V is VERY demanding.)

There is also a question about the extent of charging that will be provided by the vehicle's alternator, and how much you will be relying on solar. If you are likely to remain stationary for more than a few days, solar is great, but if you expect to run the engine for a few hours on most days then a dc-dc charger driven by the alternator may be a better option. (We have 200 Ah or storage, similar load to yours, and both a good solar setup and a dc-dc charger. With our travel pattern, rarely in the one spot for more than 2 or 3 days, the dc-dc charger does most of the work.)

You may findElectricity for Camping a useful read.

We can probably be more helpful if you let us know how your 2 batteries are connected and used - especially, are they connected together as if a single big battery, which one/both drives the Engel, are you also charging from the vehicle, are you aiming to rely on solar for extended periods ?.... and...... please tell us you aren't running the 3 way fridge from battery!


J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
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AnswerID: 518712

Follow Up By: Twinkles - Wednesday, Sep 25, 2013 at 14:19

Wednesday, Sep 25, 2013 at 14:19
Hi John & Val. I haven't actually got the camper yet. Just getting prepared. Thanks for the info on 3 way fridges.
FollowupID: 798626

Reply By: Member - Peter E7 - Wednesday, Sep 25, 2013 at 10:00

Wednesday, Sep 25, 2013 at 10:00
Have a look at this website
Has a lot a technical info on sizing panels
AnswerID: 518713

Reply By: olcoolone - Saturday, Sep 28, 2013 at 10:11

Saturday, Sep 28, 2013 at 10:11
It's good to see so many are calculation capacity for solar panels based on 100% efficiency and maximum peak sunlight hours.

In the real world it's so different.

Go the biggest you can, bigger is better.

AnswerID: 518854

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