12v power for CPAP machine.

For boondocking, I tried to set up a small inverter to provide power for my wife's CPAP machine in our RB setup. Actually, I tried a 200w, then a 400w inverter, with the same result: the inverter faulted out with a low battery voltage warning. The machine is only rated at 125w, so I am thinking that the starting load is quite a bit higher, and the voltage drop from the front to back of the trailer might be enough under high transient loads to cause the fault. The inverters work fine up front...I use the bigger one to run the TV and DVD player.



A friend suggested that adding a small marine AGM battery near the inverter might help buffer the load, and serve as emergency backup should we lose power. I'm not sure hooking it directly to the 12v lines is good since I don't have any way to control the charging current. Right now my base plan is to buy a small 4 stage charger ($30 on Amazon) and charge the battery from shore power.



The ideal solution would be a relatively small and cheap 12v converter that would manage the charging, whether from 12v or 110vac. Does something like this exist? Any better ideas how to do this?



FYI..I can buy a solution from the CPAP machine company that does battery backup....$500. I'm highly motivated to find another way!



Thanks





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