DC to DC charger - impact on ‘donor’ battery

[stranded]

Thanks to lots of invaluable advice and encouragement on a separate thread I am now ready to switch my service bank over to LifePo4.

I am just having a bit of a wobble over how the dc to dc charger from the start to the house bank actually works. I assume that for the start battery to fulfil its function to limit the alternator draw, the alternator current must go into the start battery and then be ‘withdrawn’ by the dc to dc charger?*

But wouldn’t that mean that the starter battery was continually being sacrificially cycled, and thus not likely to last long. As this is a pretty settled technology I assume I’m wrong, but I have read one US thread where someone claimed they trashed their start battery in well less than a year iirc. TBH the sacrifice itself isn’t too worrying - start batteries are cheap - but if it is true I think I’d rather run an entirely sacrificial 4th bank and keep the start battery clean.

* Otherwise why could not the dc to dc charger just be connected direct to the about to become redundant alternator splitter house bank output?

 

[Daverw]

I would have thought that both the starter battery and the dc to dc charger would be seen by the alternator as a parallel load so do different in have a vsr in LA systems

 

[lektran]

The alternator needs to supply more than the charger draws, if that condition is met then you shouldn't have any issues. Cycling would occur if the alternator can't keep up, the charger would drain the start battery until the it senses the input voltage is too low and then shut down. Eventually the start battery recovers and the cycle starts again.

 

[PaulRainbow]

The starter battery doesn't get continually cycled, as such, the alternator will raise the terminal voltage and charge the starter battery, if need be, whilst also "sending" current to the DC-DC charger. Victron (most ?) chargers will shut down if the input voltage falls below a set level and can also be configured to only operate whilst the engine is running.

 

[stranded]

This is helpful and reassuring, thank you. I don’t suppose anyone knows off the top of their head how my Sterling ProSplitR alternator splitter will cope with the combined draw of the start battery and the dc to dc charger. As the dc to dc will be pulling 14.2 amps, will the ProSplit, which presumably can’t tell the difference, assume the start battery is heavily depleted and prevent charge going to the bow battery until the start battery is apparently charged enough - which it never will be - or not for a very long time?

 

[William_H]

I think you will find and I may be wrong that th splitter simply makes a connection from alternator to batteries which in effect look like a hard wired connections alternator to each battey but not battery to battery. Thus if one battery is low most of the current will go to that because the voltage will be pulled down by that charge current. Hopefully the alternator will be able to provide that current to the battery and dc to dc charger as well as any other battery on the splitter. If not then voltage will fall and so charge current gets less. The dc to dc charge will in fact take more current if voltage falls but will turn off if voltage falls too far. I would expect this turn off/on voltage to be around the 13 volts ie that of a battery being charged so no risk ton discharging the battery.
The hype about the splitter charging one battery lowest first is simply that the ow battery will take more current. No magic. ol'will