Redirect solar DC energy to heater when temperature is too low to charge lithium batteries?

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craigh-1 asked Dec 29, '21

Redirect solar DC energy to heater when temperature is too low to charge lithium batteries?

I have a remote cabin with 5 kW solar, a 150V/100 Victron MPPT and Victron Lithium batteries with a 5 degree C charging cut off. When the temperature drops it's often sunny out, but the batteries won't charge due to the low temperature. Is it possible to redirect the MPPT power to some form of DC heater? Does the MPPT output turn off at 5C or is it the battery BMS that tells it to shut down? I'd like to redirect the DC power to warm up the cabin and ideally get it over the 5C cutoff to start recharging the batteries.

Thanks!

MPPT SmartSolar Lithium Battery

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Answer 1 · 2021-12-30T03:38:26Z

snoobler answered · Dec 30, 2021 at 03:38 AM

The MPPT can have low temp charge protection enabled or disabled. It is important that the MPPT receive temp data from a BMV, smartshunt or a smart battery sense connected through a VE.Smart network. If you have a GX device, then the BMV or smartshunt can feed temp data to the system through VE.Direct.

If enabled, the MPPT will clearly not charge until the temperature raises, so you would have to disable MPPT low temp charge protection to be able to heat from solar.

Some BMS do not have low temp charge protection. You need to determine that for yourself.

You would need to devise your own control circuit and heating system to accomplish your goal.

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Answer 2 · 2021-12-31T06:28:08Z

craigh-1 answered · Dec 31, 2021 at 06:28 AM

Ok thanks. I've been trying to figure out a Victron solution but there doesn't seem to be a workable design. I've seen a lot of research into water heater power sinks but those are typically after fully charging batteries and using AC power. I think a resistive radiator powered straight from the panels + a DC power contactor to switch it off once it's warm enough for the batteries to charge might work.

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Answer 3 · 2021-12-31T16:09:08Z

hjohnson answered · Dec 31, 2021 at 04:09 PM

What I would consider doing is insulating the heck out of your battery box, then adding a little thermostatically controlled heating to the box. Just enough to keep it at, say, 10C. Given 5cm or so off polystyrene, it shouldn’t take much to keep your batteries warm enough. There are heating pads out there that are gentle enough so as to not cook the batteries.

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nebulight commented · Dec 31, 2021 at 07:28 PM

On top of this you could also wire it up to a timer to not turn on at night since you wont' be charging at night. However if you did this you'd be wasting part of the morning's solar as the batteries would need to heat up before charging would start. But if insulated well enough, the power usage over night would probably be minimal.

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craigh-1 nebulight commented · Jan 01, 2022 at 11:06 PM

I've tried this but the power then must come from the batteries and that drains them further while crossing my fingers they warm up enough to start charging. They didn't get above the 5C limit and ended up running down completely before I could get there to warm up the cabin on a sunny day. They've got a lot of thermal mass so they take quite a while. We also get many dark sunless days in a row so it's too risky to drain them trying to heat themselves. I've got 10 kWh total so it doesn't take long to drain if used as a heater. I think using the solar DC voltage is the best option if I can figure out how to safely switch it on/off remotely.

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Answer 4 · 2021-12-31T17:36:03Z

rdcat answered · Dec 31, 2021 at 05:36 PM

Use L2 output switched based on temperature.

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snoobler commented · Jan 01, 2022 at 11:37 PM

Are you referring to load ports? If so, 150/100 doesn't have them.

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Answer 5 · 2022-05-01T16:31:33Z

nbezanson answered · May 01, 2022 at 04:31 PM

Yes, this is a feature request!

I just found this thread because I'm looking for the exact same thing: Use the load output to drive a heater. When it's so cold the battery charging is disabled, but the sun is up, take the energy from the solar input and send it directly to the load (heater) instead. Yes it'd be basically the thermostatic heater already described, but with the additional knowledge of when the sun is up, so it'd never drain the batteries into the heater -- only solar energy could be used for heating.

The SmartSolar load output functions (streetlighting, user configurable, etc) are already wonderful and they're a major selling point for me, but adding a low-temp and sun-up function would make them even more useful. (Also, letting the Tx port have a separate config from the Load port, so two separate functions could drive the two outputs.) Then I could use the Load terminals for the battery heater, and the Tx port to signal my BatteryProtect which handles the larger loads.

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mihaitaiosub commented · Aug 23, 2022 at 10:18 AM

But what can you do when there is no load output, physically? In the larger versions of smart chargers, this output does not physically exist.

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Answer 6 · 2022-08-20T17:21:57Z

dandenson answered · Aug 20, 2022 at 05:21 PM

Like nbezanson, I found this for the same need.

I'd like to be able to use all victron for charging and 'BMS' duties with the need to heat up the battery box if it's too cold to charge. Otherwise I have to put this all on the battery with a separate BMS and disable the low temp charging protections.

It would be nice to have the MPPT go into 'low output mode' or 'matched output' and instead of outputting at charge current, read the battery volts at the terminals and then output at .1v higher than that. That way you'd never try to charge but you'd be able to power your heater from solar. Trigger a smart relay to 'on' during this time and have the heater on that relay.

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Answer 7 · 2023-01-21T21:56:01Z

stewartpe answered · Jan 21, 2023 at 09:56 PM

Dittos on the MPPT feature request!

Since LiFePO4 batteries have their own BMS, you can connect your loads directly to the battery. The BMS will handle low voltage and temperature cutouts.

This frees up the Load terminals to run the heater. Here's the logic needed:

1. Temperature is less than 40°F (adjustable) and
2. Voltage is at or near setpoint. This indicates that either the battery isn't charging, or that there is extra power available to heat the battery.

Last night, the temperature dropped into the 20's. Here's a voltage chart from one site without a heated battery:

-Around 04:30, the BMS switched off charge capability. The battery voltage dropped due to the insertion of an outbound diode.
-Around 07:30, the sun came up. The voltage immediately popped up to setpoint. This is when the heater should be switched on.
-Around 11:00, the battery warmed up enough to begin charging. This is when the heater should be switched off.

lifepo4-charge-curve-230121.png (7.8 KiB)

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