can I get low temp protection from only 1 battery in series?

[ryankshaw]

I just got done building my first solar system in my travel trailer. I used 2 of the cheaper LiTime 12v batteries in a 24v series. I bought them before I even knew about how you shouldn't allow Lithium batteries to be charged below freezing and why you'd need low temp protection. so that's what I have and where I'm at.

but now, going into winter I know I need to do something about it. here are some options I've thought of:

1. use a holding tank heater pad placed directly under the 2 batteries and turn it on all winter so if it drops below freezing outside, it heats up the batteries enough to stay above freezing.
   • cons: what if the thermostat in the heating pad fails to turn on? what if battery gets completely drained to the point that the heating pad isn't powered?
2. wrap the batteries in some kind of insulation and cross my fingers that the batteries hidden in that insulation inside my trailer never get below freezing.
   • I don't think I can get away with this, I'm sure some night here in utah it will get cold enough that by morning when the sun comes out, the batteries will still be below freezing
3. completely turn of everything all winter.
   • cons: I was hoping to run an extension cord from my trailer that is parked next to my garage and power all the loads in my garage from my trailer all through the winter. If I turn it off, can't do that
4. buy some other device I can put somewhere either on the wire coming in from the panels or on the wires connecting the batteries that has some kind of temperature sensor and can break the circuit if a certain temp gets hit.
   • I don't know of such a thing? does it exist?

but here's my latest thought:

what if I buy another battery, but this time one of the LiTimes that does have the low temp cutoff built in? and wire that up to the others in series?

(so either replace just one of the 2 batteries so one is the non-low-temp version and the other is the low-temp-cutoff version. or buy 2 more so there is 4 batteries in total 2s2p and only some of them have the low temp cutoff). I believe the batteries are similar enough otherwise that it would be ok to put them in series. Would that break the circuit and effectively give me low temp protection for the 2 batteries I already have?

or what would you do if you were me and needed to add low temp protection to a system you already have?

 

[rodrick]

Using 1 battery with low temperature disconnect would work not the best option thought, does your scc have a temperature sensor that you could use, also if you go with heated batteries as a replacement make sure that the voltage matches your system don’t series connect heated batteries you will fight imbalance issues constantly

 

[MisterSandals]

but now, going into winter I know I need to do something about it. here are some options I've thought of:

What other equipment do you have in your system?

You have a good understanding of the issues and pros/cons of your identified solutions. With a better understanding of your entire system, maybe there is a better option.

 

[mikefitz]

Buy some other device

WH7016C 12V Automatic Digital Temperature Controller Thermostat Control Switch | eBay

Delay protection function. Upper and lower limits of temperature can be set. Storage Temperature:-10~60 degree. With large and clear LCD display for better readability. Cable Length: 100cm (Approx.).

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10pcs Temperature Control Switch Thermostat 5°C N.O. | eBay

Reminder: Be sure to use your product as directed;. - We hope you enjoy using our item.

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KSD301 Temperature Switch Control Sensor Thermal Thermostat 5°F-86°F/-15°C-30°C | eBay

KSD301 Thermostat Temperature Thermal Switch Normally Open/Closed NO/NC. Automatic reset. Application for household electric appliances, PC, microwave ovens, Irons, refrigerators, electronic oven, heating unit, coffeepot, water heater, and many more.

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Some solar controllers have a control input that could disable charging, Victron controllers have this function .
The temperature sensors may need a power relay suitable to control solar panel feed

 

[ryankshaw]

What other equipment do you have in your system?

I have 4 of these panels (got them used for super cheap).
going into an MPP pip-2024lv-mk all-in-one (which does not have any low temp protection)
hooked up to the 2 no-low-temp-protection LiTime 12v batteries I mentioned above wired in 24v series
and for what it's worth, I do have 12v available because I also added a 24 to 12 v step down buck converter to power all my 12v stuff in the trailer

so unfortunately the charge controller (built in to the all in one) doesn't have low temp protection either

 

[ryankshaw]

Using 1 battery with low temperature disconnect would work not the best option though

just curious? why would it not be a good option? would it work but not be ideal or would it just not work?

and as far as your other question, unfortunately my scc does not have a temperature control on it

 

[ryankshaw]

Doing more research I also found this video:

Basically, his idea is to use a cheap-but-low-amp W1209 temperature relay to turn on a higher-amp solid state relay

I think that might possibly work if I put that between the solar input and the all-in-one since the max voltage PV open circuit voltage would be ~145VDC

so that could be a possibility, what do y'all think about that? or can anyone think of something more straightforward & reliable?

 

[cs1234]

but here's my latest thought:

what if I buy another battery, but this time one of the LiTimes that does have the low temp cutoff built in? and wire that up to the others in series?

(so either replace just one of the 2 batteries so one is the non-low-temp version and the other is the low-temp-cutoff version. or buy 2 more so there is 4 batteries in total 2s2p and only some of them have the low temp cutoff). I believe the batteries are similar enough otherwise that it would be ok to put them in series. Would that break the circuit and effectively give me low temp protection for the 2 batteries I already have?

or what would you do if you were me and needed to add low temp protection to a system you already have?

In my experience with 12v lifepo4 batteries in series, it doesn't break the circuit. The remaining 12v battery that hasn't disconnected for low temp protection still sees the voltage from the other battery (which has only disconnected charging, not output) and will continue to charge even though the other one has blocked charging.

 

[Hedges]

If you connect two batteries in series and one stops current, I think zero current flows through the other, as OP also thinks.

Both BMS need to be compatible with 24V configuration of two in series. Whichever disconnects for any reason will experience 24V across its open FETs.

In my experience with 12v lifepo4 batteries in series, it doesn't break the circuit. The remaining 12v battery that hasn't disconnected for low temp protection still sees the voltage from the other battery (which has only disconnected charging, not output) and will continue to charge even though the other one has blocked charging.

Have you measured that, current continues to flow?
How could it, nothing to carry the current?

 

[cs1234]

I use 12v lifepo4 batteries with bluetooth in series. I've seen batteries in the series engage their charging disconnect for high voltage many times. The remaining batteries in the series continue charging, their bluetooth shows them receiving the same current as before the disconnect, plus the additional current that the other battery is now not taking.

I don't know how it works, only that that is what happens.

Remember, they have separate charging / discharging disconnects.

Edit.. I'll see if I can figure out what button to push in the bluetooth software to block charging to simulate the condition if nobody wants to beat me to the experiment.

 

[Hedges]

I use 12v lifepo4 batteries with bluetooth in series. I've seen batteries in the series engage their charging disconnect for high voltage many times. The remaining batteries in the series continue charging, their bluetooth shows them receiving the same current as before the disconnect, plus the additional current that the other battery is now not taking.

Two 12V batteries in series, for 24V.

If SCC was delivering 24V at 10A, each 12V battery was getting 12V at 10A.
If one disconnects, you say the other gets the current the first no longer took? 20A not 10A?

The SCC might be willing to convert PV input to the lower voltage.
The BMS can disconnect cell+ from battery+ (or is that cell- from battery-), but what produces the short circuit from battery+ to battery- to complete the circuit from SCC to other battery?

If you connected the batteries in parallel the sure, if one disconnects the other gets double current.
But I don't see how that works in series.

 

[rodrick]

just curious? why would it not be a good option? would it work but not be ideal or would it just not work?

and as far as your other question, unfortunately my scc does not have a temperature control on it

My concern is the un protected battery could drop below 32 and the other could still be above by a degree or two, you have no protection on the basic battery, this could possibly happen due to its location maybe closer to a outside wall or a draft hitting it first or the other battery warms up first and starts charging just things that could happen

I think when one of the bms goes into protection mode it should break the circuit, when in series if one opens you should have no power so it should work but not ideal