hybrid Battery Maintenance charging unit.....info

Hello all. I have searched and found Grid Chargers available on flea bay for $375. Prolong chargers for $750 and DIY for $100. I have a friend who has a 2010 blown engine and is saving to buy a Hybrid pit engine and have it installed. During that down time I am convincing him he needs to keep the 12V battery and Hybrid battery alive with trickle charging. The 12V battery can be pulled from the car and put on a........(insert your recommended link) here....But for the Hybrid Battery is there a less expensive option than $375 other than DIY? This guy might have trouble even installing a harness for a MAXX VOLTS Grid charger...unless I can get him recommended help. Anyone on here build and sell Grid charging units? I know the Maxx Charger doesn't discharge but the car will do that over time..on its own...this could be a year long process.....just trying to keep his car out of the scrap yard. To make things even more fun. He is 800 miles from me so I can't assist in person. So yeah this has been fun.