We are caravanners who predominantly camp off grid.
During winter we find our 90Ah Gel LA battery alone, despite its solar array has inadequate energy capacity.
With a minimum of hassle and outlay I want to top it up, I do so but not IMO by an elegant technique.
I presently use a portable 12 VDC 100Ah LiFePo4 trolling battery I have.
For that I cobble up an inverter and CTEK charger but realise this is likely to be a crude poor efficiency solution.
Would a DC to DC converter, the Victron 9Amp be a better, modest outlay solution?
Has that inherently the right charging algorithm embedded to “kindly” recharge the Gel battery?
Is there a feature to limit the over discharging of the LiFePo4 battery, or is that only met by it falling back on its own BMS?
Here the thoughts are leaving it overnight when needed, to do the transfer. Then I can, if needed recharge the LiFePo4 via my vehicle etc, however more than doubling my energy storage has proved adequate enough so far, albeit in the method described.
I don’t want to change much of what is an adequate system, nor manhandle a battery physical change out; just want to as said hassle free top what I have up on the limited times I need to. Prepared to invest in what a modest capacity DC to DC converter costs if that offers an elegant solution.
I assume that you are thinking of the Victron Orion isolated converter 12/12-9.
This is a power supply which has a fixed output voltage rather than being a true charger. You could theoretically set the output voltage to something like 13.0V to 13.2V which would keep the gel battery full but not overcharge it if the load is low. Then if the load increases and the solar can not supply then the voltage will drop and the converter will then take up the demand followed by the battery. With such a low voltage from the converter it will not be good at recharging the battery if the battery is discharged. If you set the converter voltage higher to get higher charging current then it will overcharge the gel battery because there is no way to go to float when it is full. Finally, the datasheet nor the manual make no mention of shutdown on low input voltage so you would rely on your lithium battery BMS.
The correct solution is an Orion Isolated charger 12/12-30 but this will be more expensive and probably have a higher capacity than you need. This has a true charger with absorption and float and the voltages can be set for gel batteries. Finally, these have low input voltage shutdown which can be configured to suit whatever residual charge you want in your lithium battery.
Thank you for your in-depth and informative reply.
What I needed to be aware of though unfortunately it makes clear my dreamed of solution is fundamentally unsound. Something best realised sooner than latter.
It was the 12/12-9 I was focusing on, and yes, the better suited one understandably comes significantly more expensive.
I can’t here, to simply ease an occasional issue. justify that level of spend. As and when needed I will continue to endure my rather messy solution.