2 batteries vs 1? (aka cold beer for cheap)

[monkeyodeath]

That is the compromise. There's nothing as expensive as trying to do something cheap. The cheapest thing to do is buy the lowest end Walmart car battery, pre-charge it at home and run it from the existing solar in camp.

Yeah, I hear that. I just think for me it's been shocking how things can snowball from "it would be cool to have a fridge in my truck"!

 

[monkeyodeath]

It depends on your usage, but a 200W solar panel in sunny weather will handle most small fridges. In my opinion you will get much better performance with a dedicated battery, ideally in your bed. You can use a removable box to allow for easy removal when not needed. A pair of golf cart batteries are great for regular usage, but as you say, they are heavy (part of why they are so reliable). If you only plan on doing the 2-3 day cycle down a few times a year, then a decent quality dual-purpose flooded lead battery in the 100AH range will likely do the trick. Alternator charging is useful, but with decent solar, it won't provide much value for camping where you are parked a lot. If you have higher power needs than just a fridge/lights, then you can consider an alternator charge circuit. Most Toyotas don't see much over 13.9V at hot idle, so the charge rates for a direct line, especially with a bit of wiring drop, are pretty low anyways.

Is solar going to do anything for me while I'm moving (on a sunny day), or is it mostly a nonfactor?

 

[john61ct]

The level of amps input from your alternator will make solar irrelevant while you are driving.

 

[monkeyodeath]

The level of amps input from your alternator will make solar irrelevant while you are driving.

I was thinking if the house battery was solar-only and not connected to the alternator.

 

[DaveInDenver]

I was thinking if the house battery was solar-only and not connected to the alternator.

In that case you'd have two choices I suppose. Figure out a way to mount the panel so you can drive with it exposed or stop enough to deploy them. You'd want to size everything to give enough margin to charge in less time and use less than all of the battery each day. You could perhaps run a 100 A-hr (it might be easier to use W-hr, so for easy numbers 1,200 W-hr) battery that you discharge 300 W-hr (25 A-hr) daily and return 600 W-hr (e.g. average 100 watts over 6 hours from your solar) every other day in camp. The problem might be if you don't have a way to expose the full 200 watts of panels enough, be that from surface area to mount them or stopping long enough to deploy them portable, or the weather is overcast for multiple days. If you're running the engine, though, you might as well use it to charge batteries when you can. The one option with that is if you use lithium, which your solar controller may already handle but would take more effort to make compatible with your truck's charging system. That's a cost/benefit balance question. If you stick with all plain lead acid you can just use jumper cables and charge the battery in a pinch if you don't want to hard wire and use an isolator.

 

[john61ct]

I was thinking if the house battery was solar-only and not connected to the alternator.

That would be silly.

Take your power wherever you can get it.

Starter needs hardly any input at all, House is the one needs all it can get.

Plug into shore power too whenever you get the chance.

 

[monkeyodeath]

Shoot, I guess I was confused about lithium.

I thought that FLA (true) deep-cycle batteries (like golf cart batteries, etc.) were the ones that were not really going to be charged by the 13.9V coming from my alternator.

I thought I'd read in another thread that lithium batteries will charge more easily on that kind of voltage. Is this wrong?

 

[luthj]

You are correct, lithium batteries will charge fine at lower voltages. Since they don't require a full 100% charge for longevity, they are well suited for this type of charging.

 

[DaveInDenver]

Shoot, I guess I was confused about lithium.

I thought that FLA (true) deep-cycle batteries (like golf cart batteries, etc.) were the ones that were not really going to be charged by the 13.9V coming from my alternator.

I thought I'd read in another thread that lithium batteries will charge more easily on that kind of voltage. Is this wrong?

The stock charging systems of our Toyotas won't fully charge lead acid batteries of any type, flooded, AGM. For most people it's not a problem because they don't really ever use much of their battery just starting and driving around. Since they are oversized for most tasks no one notices the slow death.

When you run a fridge or the guys with boom-boom stereos or anyone else who relies on the full capacity being available repeatably this becomes a problem since you never reach 100% even on a brand new battery and with each cycle you lose capacity.

But this is a separate issue from LiFePO4. You are correct that the charging system on our trucks works quite acceptably for LFP.

The main problem with just sticking one on a lead acid charging system is turning it off when it's done charging (you don't float them) and controlling the current so nothing overheats, which is why LFP batteries often have a BMS built-in to handle the details.

 

[shade]

As usual, very good advice was already given. I'll only add that a good jump pack should go with you, especially as long as you're running the fridge off the start battery.

 

[john61ct]

The correct charging profile for every decent battery is clearly spec'd by the maker.

Yes can generalize by type, but it does vary.

The LFP specs are too high for good longevity, stressful maximums, bad for them to sit Full.

But lead really does need to get to 100% Full as often as possible, so going a bit higher than spec is better than lower.

 

[Swiftone]

I have posted this in the past, but a 20 AH LiFeP04 plus a 100 watt panel has kept my sons CF-35 at 39 degrees 24/7 over the past month or so in Southern CA heat. If you already have the panel, a 20 ah LFP battery is a not much more...flat roof mount panel, cheap Renogy PWM 10 amp controller and the fridge.

 

[Peter_n_Margaret]

The cheapest solution is to simply run the fridge from the crank battery via a low voltage cut out ($10) that ensures that there is always enough power left to start the vehicle. The solar can charge the crank along with the alternator.
Cheers,
Peter
OKA196 motorhome

 

[monkeyodeath]

The stock charging systems of our Toyotas won't fully charge lead acid batteries of any type, flooded, AGM. For most people it's not a problem because they don't really ever use much of their battery just starting and driving around. Since they are oversized for most tasks no one notices the slow death.

When you run a fridge or the guys with boom-boom stereos or anyone else who relies on the full capacity being available repeatably this becomes a problem since you never reach 100% even on a brand new battery and with each cycle you lose capacity.

But this is a separate issue from LiFePO4. You are correct that the charging system on our trucks works quite acceptably for LFP.

The main problem with just sticking one on a lead acid charging system is turning it off when it's done charging (you don't float them) and controlling the current so nothing overheats, which is why LFP batteries often have a BMS built-in to handle the details.

Thanks for the great explanation! Makes a lot of sense. Is it better to look for a lithium battery that has onboard circuitry to handle charging, or do you want to get a separate unit?

 

[monkeyodeath]

The correct charging profile for every decent battery is clearly spec'd by the maker.

Yes can generalize by type, but it does vary.

The LFP specs are too high for good longevity, stressful maximums, bad for them to sit Full.

But lead really does need to get to 100% Full as often as possible, so going a bit higher than spec is better than lower.

Sounds like, were I to use a flooded or AGM battery, I'd want to put it on a real battery charger and tender when I get home from a trip. Might be another argument for using an external battery box.