What happens to the solar controller when the lithium battery BMS halts charging due to low temp?

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stillRV

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Hi, I am considering upgrading my motorhome house batteries to lithium and adding solar. I have a question maybe someone on here can answer.

I know most lithium batteries have a built-in BMS that disables charging at freezing temperatures. The manuals of solar controllers say that you should always hook up a battery on the output before connecting solar panels to the input of the controller. So what happens in freezing temps when the battery BMS stops accepting a charge? Now the solar panels are connected to the solar controller but effectively there is no battery hooked up.

I am considering a cheaper lithium battery that does not have a low temp cut off. I thought this would be a big hassle, but it seems like I may have a low-temp hassle anyway if I have to be sure to disconnect the solar panels when the outside temp dips below freezing?

Thanks.
 

jkane

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Denver, CO
First, most LiFePo batteries do not have low temperature protection. The big names do, so check to be sure (Youtube Will Prowse is a good resource).

My understanding about the Solar Charge Controllers is that they need to see the battery first so they can know the voltage of the battery packs (12V, 24V, ...). Once the SCC knows the voltage it is then "safe". I have not tested this yet, but as I understand it, most batteries still allow you to draw current even when the low temp protection has been tripped. The protection is to stop charging at a low temperature. The SCC will treat this just like the battery is full, the battery is no longer accepting charge current. All that should be happening is that you are losing some charge that could have been pushed into the battery bank.

If the batteries are in the living quarters, then you are likely not using any power (I would not want to be in there at an internal temperature of freezing). If the batteries are in another compartment, you could look at a heating pad (and burning up some of that solar energy) to keep the batteries above the low temp protection point.

In any event, you do not have to disconnect the solar panels. A proper battery will protect itself and all you miss out on is some charging.
 

Gary RV_Wizard

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If the BMS shuts itself off, there is no place for the solar controller (or converter or whatever) to send power, so it just sits there idle. Nothing happens because no power is flowing anywhere.

This is like asking what happens when I unplug a device from a wall socket? The answer is nothing - there is electrical potential at the outlet, but "potential" is all that it is. No power flows and nothing happens.
 

John From Detroit

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Davison Michigan
First low temp is unlikely while charging
2nd the Solar controller will see an open circuit or the RV's parasitic loads and not be bothered one little bit.. It will basically Say "Oh goody, Light Duty" (Take that two ways. Light load or Only light bulbs as load)
 

stillRV

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If the BMS shuts itself off, there is no place for the solar controller (or converter or whatever) to send power, so it just sits there idle. Nothing happens because no power is flowing anywhere.

This is like asking what happens when I unplug a device from a wall socket? The answer is nothing - there is electrical potential at the outlet, but "potential" is all that it is. No power flows and nothing happens.

Yes this makes sense to me. Which is why I was surprised that the manual says to never connect it to solar without connecting it to a battery.
 

Gary RV_Wizard

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Which is why I was surprised that the manual says to never connect it to solar without connecting it to a battery.
An excess of caution, methinks. If you are the manufacturer of a product like that, you worry about all kinds of bizarre issues that too-often occur out in the wild. A dangling wire shorts to ground, or somebody thinks they can directly plug their inverter to the controller terminals with no battery. The engineers want you to configure it the way they designed it to work cause that's what they tested in the lab.
 

Obiwan Canoli

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I understand there are now LiPo's that are designed to charge at below freezing temps... further, you could add a system that keeps the batteries warm enough - above freezing - so that this issue isn't an issue... or, you could house your LiPo's in an area of the MH that would not get below freezing temps...
 
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