Monday, Jan 27, 2014 at 12:08
Hi Eaglefree,
Battery sizing is related to charging opportunities, so first my thoughts on that -
Like Frank, I wondered if solar panel/s might be a good option for you, especially the ones Frank has referred to. There is a catch 22 compromise here though - at first sight, fitting panels to the top of your rig sounds good - but - that will require that you park in the sun, which will add to the fridge's load. Being horizontal is suboptimal too. I think your panels would be best if removable, so the trailer can be in the shade with the panels in the sun and correctly oriented. If that's possible, then adding some light weight panels sounds very good.
How big should the battery be ? Without solar you probably need about 40Ah to give 2 days use. Ideally this shouldn't run your battery down more than 2/3, so you'd need at least a 60Ah battery. (Let's assume that you can fully recharge it from the alternator on the next travel day. Recharging is a whole new area of concern, so easier to make that assumption, while not forgetting that it can come back to bite!)
With solar, assuming 7 hours good sun per day, (assumptions make it much easier!) to meet your daily needs you need to generate about 20-25 Ah, ie a charge rate of about 3.5 amps, which would call for a 60W panel with a PWM controller, or 50W with an MPPT controller. With this idealised system you really only need enough battery capacity to deal with the other 17 hours, so a 25 Ah battery should be adequate. Being reasonably conservative, lets say a 50W panel and a 40 Ah battery should be good. (Sorry about all the vague numbers and simplistic analysis, but there are many unknowns and it's probably best to be conservative and aware of the underlying assumptions.) These considerations rely on the solar panels to provide all charging. In fact the alternator will probably also be a major contributor.
Being able to use the auxiliary battery as a starter sounds a very good idea. It also suggests thinking about using a marine style battery, rather than an AGM deep cycle type. These are suitable for both cranking and deep cycle applications. Rather than the household chores, the ability to crank the vehicle will probably decide the minimum size of the battery.
HTH
Cheers
John | J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein
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