Roof mounted solar or alternator charging?

Submitted: Saturday, Mar 09, 2013 at 22:14
ThreadID: 100997 Views:2386 Replies:5 FollowUps:2
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I'm looking to power a 52L Bushman fridge that I usually run at 1 degree C. My style of camping is usually about 5 nights in the same place with about 30 minutes of driving per day. I'm currently weighing up whether to go for a (roof mounted) solar or alternator charged auxilary battery system. I already have a newish 105AH AGM, but unfortunately this won't fit under the bonnet of my Prado 150 series. So if I were to go for solar, I would not need to buy a new battery. If I go for an alternator charged dual battery system, I would need to get a smaller battery to get it into the bonnet. I also use the third row seats fairly regularly, so prefer to be able to easily remove the fridge and battery if it is stowed in the third row.

Any thoughts on which is the better setup? And which would be easier on the wallet? I do know that I'll be up for about $1k for a dual battery setup (alternator option).
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Reply By: Tim - Saturday, Mar 09, 2013 at 23:52

Saturday, Mar 09, 2013 at 23:52
A roof mount solar panel means you will either have to park your car in the sunlight and or move it through out the day to keep the sun on it in,order to get maximum use out of the panels.Parking in the sun means your fridge will be working harder.
I have the same issue, big battery from a 90 series which now doesn't fit in the 150 so,I will be getting a smaller battery for under the bonnet and chucking the big one in the camper. Small one is alternator charged and big one will manly be solar charged but also alternator when the trailer is on the move as well.
AnswerID: 506442

Follow Up By: Member - John and Val - Monday, Mar 11, 2013 at 12:01

Monday, Mar 11, 2013 at 12:01
Hi Tim,

A rooftop mounted panel doesn't mean it has to be permanently mounted. We have 2 panels mounted on the top of our Troopy but both are easily removed if we want to park in the shade and have the panels working to capacity. Not hard to do.


J and V
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FollowupID: 783539

Reply By: Ross M - Sunday, Mar 10, 2013 at 00:26

Sunday, Mar 10, 2013 at 00:26
If you already have the battery and just use a continuous solenoid , switched on by you, it won't cost anywhere near $1000. Especially if you do much of the fitting work and running cables etc. Time consuming but not really hard.

If your fridge uses a conservative 3amps while running and it runs for 50% of the time then that equates to 36AH/day. 2 days equals around 75% of the batteries total AH rating charge.

AGM batteries don't like a fast charge and a 105aAH is about 20 amps max charge rate, less in hot weather when the battery may already be 32C ambient. AGM is no good under bonnet anyway as they are not heat tolerant at all.

If you run your vehicle for 30min each day, although in theory the alternator has the capacity to charge it up the AGM max limit of 20Amps charge rate will mean, either the charge of 20 AH equates to 10AH of charge delivered to the battery in that time frame.
It will probably force more into the battery in that time and so will shorten the batteries lifespan.

Many people say to have big thick cables to charge batteries and while this is true, if the cables are not as thick as usually recommended they do offer some small resistance to heavy current flow, then it, in a way self regulates when a partially discharged AGM is in use and still allows the battery to attain full voltage if it is applied to it. If not and AGM but a Marine battery, then the cable size can be big/suitable and they charge more quickly too.
This situation is the same for a manual or a voltage sensitive (VSR) solenoid.
The charge time is not long though.

A DC/DC system will charge the AGM, or any battery, but it too takes time which isn't available because of the run time.

Solar will add to the battery and supply a suitable topping up/maintenance charge if it's panel wattage has large enough output capacity.

Presuming full sun, a 120Watt panel will give around 7amps /hour x say 8hours so 56AH/day. This would go close to providing the needs of the fridge and extend it's usable time. You will get more on longer days and less or nearly nil on cloudy heavily overcast days. Lot of variables in the conditions and situations it is used in.

Some may not agree with all this but it works for me.

Ross M
AnswerID: 506446

Follow Up By: garbage - Monday, Mar 11, 2013 at 11:46

Monday, Mar 11, 2013 at 11:46
Thanks for the very detailed reply Ross! Are your calculations for the 120 watt panel assuming horizontal mounting on the roof? Also, how do most people wire the solar panel into the main cabin of the car?
FollowupID: 783536

Reply By: Member - graeme W (WA) - Sunday, Mar 10, 2013 at 04:40

Sunday, Mar 10, 2013 at 04:40
Not sure if the prado is similar to the hilux for the auxillary battery position but if it is be carefull with anything heavier that a 65amp agm in the engine bay.My 100 amp agm cracked the wheel arch housing because the auxillary battery tray is badly designed.Fixed that up and now run a 65amp agm in the engine bay and 120 amp agm under the rear tray.Run a 60 litre trailblazer and a 80 litre waeco which are charged through a solar panel on the canopy roof with a folding one for backup and for extended stays in the shade use a 1kva honda running a 40 amp xantrex smart charger.The purists will say 40 amps is too much but the batteries are nearly 7 years old now and still work like new.Same for an agm under the bonnet no problems and the one under my tray is on its side as well.Going to add another panel to the canopy roof to save carrying the folding one.
cheers Graeme.
AnswerID: 506450

Reply By: pepper2 - Sunday, Mar 10, 2013 at 08:36

Sunday, Mar 10, 2013 at 08:36
I have exactly that fridge permanently running in my patrol,set at minus 5 c,i have two wet cell batteries (not agm) connected in parallel (no electronic isolator),i have a manual isolating switch that i don't use often.
The batteries are charged by 2 x 80w solar panels mounted to the roof with a mppt regulator.
This set up runs everything,fridge,lights radios etc perfectly,the longest the vehicle has been parked without the engine running is 3 weeks parked outside my home while i used a company vehicle for transport.
Remember that the bushman has a low voltage cut out set up(that can be varied) mine is set to the lowest available setting,even if your dual batteries discharge to this level you can still start the vehicle with the batteries wired as above.
When charging with solar you get two other benefits 1.more energy charged into the batteries than an alternator and 2.much longer battery life.
Note that when solar panels are mounted horizontally they perform at approx 80% of their rated output,this can be better than having free standing panels on the ground remote from the vehicle as you have to frequently move the panels to maintain max charge as the sun moves or you lose efficiency.
I also have an additional Anderson plug from the panels that allows me to top up my boats starting battery occasionally.
This set up is fail proof,simple and much cheaper on your wallet.
AnswerID: 506455

Reply By: Member - Nolo (Brisbane) - Wednesday, Mar 13, 2013 at 09:46

Wednesday, Mar 13, 2013 at 09:46
You may be interested in this.
Prado battery options
Business owner has a Prado and supplies accessories for this vehicle specifics. He also does a nifty battery box (Flyer) as an alternative to under bonnet installation.
AnswerID: 506689

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