Do I NEED a DC/DC charger?

[larry9000]

When I switched to BB lithium I could read the charging/discharging value through the Victron app while driving. If I reved the motor it would charge the battery, but at normal RPM while driving the BB was discharging! The reason is that lithium has a slightly higher battery voltage than the lead acid battery in the tow vehicle. So when you connect them together current flows from the BB to the TV battery- it is literally charging the TV battery! I was going on a trip in another day, so I disconnected the charging wire from the TV inside the Airstream. So the TV would neither charge or discharge the BB. I relied on the converter and solar to charge the BB for this first camping trip. Later I installed a DC-DC converter to allow the TV to charge it too.

The reason it would still charge the BB when revving the motor is that the alternator output increases with RPM.

If it were me, I’d avoid this whole scenario by disconnecting the TV charge line, as I did. Or add a DC-DC converter which not only charges the BB, but prevents this reverse flow too.

[DCPAS]

Originally Posted by larry9000

When I switched to BB lithium I could read the charging/discharging value through the Victron app while driving. If I reved the motor it would charge the battery, but at normal RPM while driving the BB was discharging! The reason is that lithium has a slightly higher battery voltage than the lead acid battery in the tow vehicle. So when you connect them together current flows from the BB to the TV battery- it is literally charging the TV battery! I was going on a trip in another day, so I disconnected the charging wire from the TV inside the Airstream. So the TV would neither charge or discharge the BB. I relied on the converter and solar to charge the BB for this first camping trip. Later I installed a DC-DC converter to allow the TV to charge it too.

The reason it would still charge the BB when revving the motor is that the alternator output increases with RPM.

If it were me, I’d avoid this whole scenario by disconnecting the TV charge line, as I did. Or add a DC-DC converter which not only charges the BB, but prevents this reverse flow too.

Interesting. I need to look at whether the batteries are discharging when connected and at speed.

[Wazbro]

Originally Posted by DCPAS

My conclusion is that the concern is real for anyone wanting to charge the Lifepo4 batteries from their alternator and simply connects a big wire from the alternator to the charger. Think boat or motorhome. Some alternators may have protection for this, but some do not. It appears that the 7 pin or a DC - DC connection is not going to cause this issue.

We have a winner. It is possible with a much more robust connection, not with the 7-pin connection.