Overkill bms charge options

[7caper7]

Ok, here's my big plan. Thanks for all that gave input. Feel free to comment!

Goal: Modification to boat that had 12v system with 6 Agm's, 110vac charger for 12v, 12v solar, 12v alternator. Replacing one agm house with lifepo4 "house" bank. Normally all agm's would connect together via Victron Cyrix's if any charging source were present. 48volt lithium battery, inverter for 110v, electric motor/generator for propulsion not modified in any way.

Modifications: I decided to disconnect/separate the 12v lithium from the agm bus. This was a eaiser way than using a dc/dc converter to protect the Engine Alternator from the high charging rate of the lithium. Limiting lithium charging via Overkill BMS did not seem viable do to voltage spikes with rapid cutoff and reconnection of bms. Charging of 12v lithium is done via 110vac charger, switched on as needed. Overkill BMS set to charge to 80% normally. Changed via bluetooth to 100% if needed. Solar,110vac charger, and alternator charges all AGM's. Victron solar controller "load" output connected to lithium and setting to allow charging only after agm's are topped off. House bus remains connected to House battery (now lithium). Hot battery bus (very low current normally) moved to the bow thruster bank which is two 100ah agm's.

Operation: At dock with ac available: 110vac charger on and all batteries charged. For storage: solar will keep agm's topped off each day and 48 and 12v lithiums turned off at 50-60%. 12v turned off via bluetooth, 48v turned off by physical switch. While at anchor (off grid): solar charges agm's then 12v lithium. If 12v lithium gets to low then just turn on 110vac charger.

I tried to design the addition of the 12v lithium house bank to be safe and operator friendly. Due to the different modes of operation this was a challenge. Skipper just needs to monitor 12v lithium and turn on 110vac charger when needed.

Comments?

Thanks again!

 

[Ampster]

I may have misinterpreted your statement that the charging voltage was being set by the BMS. My preference is to set a conservative CV setting on my charger and let the BMS be the last resort fail safe for charging and discharging cutoff. That avoids shocking the components with voltage spikes. Can you adjust the setting on your solar charge controller or the 120 volt AC charger?
There is some effiency lost going from 12v DC to 120 volt AC and back to 12Volt DC, but the convenience of charging from shore power may offset that.

 

[7caper7]

I may have misinterpreted your statement that the charging voltage was being set by the BMS. My preference is to set a conservative CV setting on my charger and let the BMS be the last resort fail safe for charging and discharging cutoff. That avoids shocking the components with voltage spikes. Can you adjust the setting on your solar charge controller or the 120 volt AC charger?
There is some effiency lost going from 12v DC to 120 volt AC and back to 12Volt DC, but the convenience of charging from shore power may offset that.

Thanks Ampster! That's my plan now. Set CV on 120vac charger and solar and let bms protect lithium. (120vac charger is powered by inverter via 48 volt lithium that has gobs of solar. Yes, some loss in 48v solar to 110vac to 12v dc, but a compromise for ease of use.)

 

[Peter Ribbens]

Ok, here's my big plan. Thanks for all that gave input. Feel free to comment!

Goal: Modification to boat that had 12v system with 6 Agm's, 110vac charger for 12v, 12v solar, 12v alternator. Replacing one agm house with lifepo4 "house" bank. Normally all agm's would connect together via Victron Cyrix's if any charging source were present. 48volt lithium battery, inverter for 110v, electric motor/generator for propulsion not modified in any way.

Modifications: I decided to disconnect/separate the 12v lithium from the agm bus. This was a eaiser way than using a dc/dc converter to protect the Engine Alternator from the high charging rate of the lithium. Limiting lithium charging via Overkill BMS did not seem viable do to voltage spikes with rapid cutoff and reconnection of bms. Charging of 12v lithium is done via 110vac charger, switched on as needed. Overkill BMS set to charge to 80% normally. Changed via bluetooth to 100% if needed. Solar,110vac charger, and alternator charges all AGM's. Victron solar controller "load" output connected to lithium and setting to allow charging only after agm's are topped off. House bus remains connected to House battery (now lithium). Hot battery bus (very low current normally) moved to the bow thruster bank which is two 100ah agm's.

Operation: At dock with ac available: 110vac charger on and all batteries charged. For storage: solar will keep agm's topped off each day and 48 and 12v lithiums turned off at 50-60%. 12v turned off via bluetooth, 48v turned off by physical switch. While at anchor (off grid): solar charges agm's then 12v lithium. If 12v lithium gets to low then just turn on 110vac charger.

I tried to design the addition of the 12v lithium house bank to be safe and operator friendly. Due to the different modes of operation this was a challenge. Skipper just needs to monitor 12v lithium and turn on 110vac charger when needed.

Comments?

Thanks again!

" Victron solar controller "load" output connected to lithium and setting to allow charging only after agm's are topped off. " Is this output controlling a relay(*) that then connects 12 VDC to your lithium (* aka an interposing relay)? I don't know the amp-rating of the Victron load output, but would certainly look into it as most lithiums can draw a lot of charging current depending on the differential voltages and SOC.

 

[7caper7]

Thanks Peter for the info! On the Victron forum I asked that question and they indicated that I wouldn't harm anything by connecting it to a battery. The solar charger will output 15 amps max or whatever the panels are producing. The issue might be that the AGM's might power the "load" and send current to the Lithium 12V. I plan to set the "load" settings to prevent that by setting the load voltage above the AGM's float voltage. I'll take the risk and give it a try and see what happens.