If you've got some Frugal DIY
genes inside of you, you can get a simple 10 amp buck boost converter, and switch it on manually when start battery charging is desired.
Takes any input voltage, and outputs a Nearly constant perfect voltage, upto 10 amp and one can limit maximum amperage by the current
I use these to speed control 24vDC fans, but have used them to EQ batteries
at 16.2v.. The heatsinks are designed to accept a 50mm computer fan which would be wise to add if approaching their amperage limits.
One can get simpler Boost converters for even cheaper, assuming they always want to boost voltage, instead of buck it. They are more efficient too.