BMS control of charger with relay (SSR?)

[ohthetrees]

Hi, I'm installing a LiFePo4 system with an REC BMS. Im trying to figure out how to control the charger, a Mastervolt Mastercharge 12-100 100 amp. The charger I have works well, but it doesn't have BMS start and stop signaling, so the only way I can think of to turn it off and on is with an SSR.

If I haven't gone terribly wrong so far, now I need to decide whether to interrupt the AC input, or the DC output. I can't just interrupt the low voltage on-off switch, because this model uses a momentary switch that I must hold down for a few seconds to turn on and off.

SSR on AC input:
Lower amperage might mean lower cost SSR? Better for charger and battery to have input power disappear than having output load disappear? Would this be a good one? SSRT-240D25 Could the REC BMS drive it directly?

SSR on DC output:
Need a bigger, more expensive SSR for 100a current. Good ones readily available from Blue Sea, like the 7701. Would it be bad for the charger to suddenly disconnect under load?

So what do you think? Relay on input AC, or output DC? Or am I missing another option?

 

[John Frum]

Does the charger have an external battery temperature sensor?
If it does, and its a negative temperature coefficient thermistor, shorting its leads will probably disable charging.
I have not tried it but it should be easy to test with a hair dryer or similar.

 

[ohthetrees]

That is a cool idea. Are battery charger thermistors typically negative coefficient? I wonder, even if they aren't, maybe shorting the thermistors leads might lead to charger stopping charge because of what it thinks is under-temperature. Worth a try!

 

[John Frum]

That is a cool idea. Are battery charger thermistors typically negative coefficient? I wonder, even if they aren't, maybe shorting the thermistors leads might lead to charger stopping charge because of what it thinks is under-temperature. Worth a try!

I don't think I've seen one that is not NTC.

 

[rickst29]

Switch on the AC input, because the proportion of power consumed by the SSR itself is much higher at low operating voltages. Why not use a mechanical relay instead?

Your charger should handle either "pulling the input power plug" (switching at AC input) or disconnecting the load from the battery bank (switching at DC output) with no problems. Just make sure that your SSR DC input scheme won't cause a lot of rapid cycling (of the charger) when the charger becomes turned off and DC voltage drops by several tenths of a Volt (from charging voltage down to discharging voltage).

I'm unsure of your goals. If you are trying to avoid charging in low temperatures, then you already have a thermostatic switch within your circuit - and you can use that switch to drive either an NC or NO mechanical relay, avoiding any need for SSR heatsinks and power wastage. But if you are trying to switch off when the battery has become "full enough", then you'll need a differential switch which turns off at charging voltage (higher voltage), but does not turn on again until your battery Voltage has dropped to a circuit-based equivalent of an MPPT controller's "Boost Return" voltage (which must be less than or equal to low-current "float" voltage).

 

[John Frum]

Switch on the AC input, because the proportion of power consumed by the SSR itself is much higher at low operating voltages. Why not use a mechanical relay instead?

Your charger should handle either "pulling the input power plug" (switching at AC input) or disconnecting the load from the battery bank (switching at DC output) with no problems. Just make sure that your SSR DC input scheme won't cause a lot of rapid cycling (of the charger) when the charger becomes turned off and DC voltage drops by several tenths of a Volt (from charging voltage down to discharging voltage).

I'm unsure of your goals. If you are trying to avoid charging in low temperatures, then you already have a thermostatic switch within your circuit - and you can use that switch to drive either an NC or NO mechanical relay, avoiding any need for SSR heatsinks and power wastage. But if you are trying to switch off when the battery has become "full enough", then you'll need a differential switch which turns off at charging voltage (higher voltage), but does not turn on again until your battery Voltage has dropped to a circuit-based equivalent of an MPPT controller's "Boost Return" voltage (which must be less than or equal to low-current "float" voltage).

Probably something functionally equivalent to this but terminating the charge by shorting the BTS leads instead of switching the ac side.

adding charge termination logic to an rv converter

I came up with a way to add automatic and configurable charge initiation and termination to this https://www.iotaengineering.com/products/detail/1188459/iota/dls-240-27v-25a-converter-and-charger/25-amp-acdc-power-converter-and-battery-charger-for-24v-battery-charging-in-240vac-applications...

diysolarforum.com

 

[ohthetrees]

Switch on the AC input, because the proportion of power consumed by the SSR itself is much higher at low operating voltages. Why not use a mechanical relay instead?

Your charger should handle either "pulling the input power plug" (switching at AC input) or disconnecting the load from the battery bank (switching at DC output) with no problems. Just make sure that your SSR DC input scheme won't cause a lot of rapid cycling (of the charger) when the charger becomes turned off and DC voltage drops by several tenths of a Volt (from charging voltage down to discharging voltage).

I'm unsure of your goals. If you are trying to avoid charging in low temperatures, then you already have a thermostatic switch within your circuit - and you can use that switch to drive either an NC or NO mechanical relay, avoiding any need for SSR heatsinks and power wastage. But if you are trying to switch off when the battery has become "full enough", then you'll need a differential switch which turns off at charging voltage (higher voltage), but does not turn on again until your battery Voltage has dropped to a circuit-based equivalent of an MPPT controller's "Boost Return" voltage (which must be less than or equal to low-current "float" voltage).

I don't know enough about relays to make an intelligent decision between SSRs and mechanical relays. I figured solid state was lower current and more reliable than keeping a coil energized, but I'm probably wrong about that.

My goal is this...

When my REC BMS decides batteries are at 100% SOC I'll send stop charge command to all my Victron stuff. I have one legacy 100amp charger I want to keep, but doesn't have any sort of BMS input or communication capability. I originally intended to use a relay on the AC input side for charge termination, but this felt very "brute force"

Someone upthread posted a cool alternative.

Basically, he suggested shorting the temp sensor leads, which would cause charger to think batteries were too hot, and stop charging. I thought I would do this by using one of the REC BMS outputs to signal a relay when I hit 100 SOC, and that relay would short the temperature input leads of the charger, which would cause the charger to think it was over temp, and halt charging. I'm just trying to figure out what hardware I'd need in addition to the REC BMS to implement this.

 

[John Frum]

Someone suggested a cool alternative in this thread:

That was me.

This is what I use for all this kind of stuff.
Price has gone up a lot.
There are probably quality alternatives by different manufacturers.
Just find this on digikey and then find all the similar/same bits.

Opto 22 DC60S3 DC Control Solid State Relay, 60 VDC, 3 Amps, 4000 VRMS Isolation Voltage : Amazon.ca: Industrial & Scientific

Opto 22 DC60S3 DC Control Solid State Relay, 60 VDC, 3 Amps, 4000 VRMS Isolation Voltage : Amazon.ca: Industrial & Scientific

www.amazon.ca