How to recharge an external battery

[Simlau]

I try to find a solution using my MME to recharge a 12 volt deep cycle battery that i use to run my electic boat motor while i am out in the wilderness where there is no electricity to plug a charger. To do so, do I have to install an inverter that will be connected to the 12 volt car system then plug a charger that will recharge the 12 volt deep cycle battery, or is this safe to just run 2 jumper cables from the car ground post an the other cable plug to the car 12 positive V post then plug it directly to the external battery, just like the way i would boost the car in case of a fail LVB. In the case i could recharge this batteryjust connecting how quick it will take to recharge I.E. how many amps have the output of the built-in MME inverter?

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[Maquis]

Your best option is probably an external inverter and charger. The internal DC to DC charger is controlled by a BMS that thinks it’s just maintaining the internal 12V battery. Connecting your external battery will only charge it very slowly and will probably confuse the internal BMS with unpredictable results.

There a a couple guys on here who know the internal BMS better than I do who may give a more definite answer.