Battery combiner sudden load question

[vroom ZOOM]

Hello everyone.

I was thinking about building a battery combiner for my boat to have the engine charge my trolling motor batteries in between fishing spots. I typically sit for about 10 min at one spot and move to another one so I was thinking I could get some juice in on those runs. I am going to be using a chip to control the connection in between the batteries. I will use five relays, one to disconnect the two TM batteries from each other, and four to connect them in parallel to the cranking battery when the engine starts charging. My question - if I have two really low batteries, and the engine is started and the relays close, will there be a sudden rush of current, and will this sudden load on the engine charging system cook it? If yes, then I can ditch two of the relays and throw in some fat transistors and ramp in the connection, but this is less desirable since those transistors love to get hot and that means energy is being lost. I heard that all the battery combiners on the market right now are basically relays, so maybe i would be fine with the relays?

BTW reason for building one and not buying one for you curious people... I will have a few other systems tied in, and having this all connected to NMEA network is also a possibility.

 

[sam am I]

So sticking (mostly) to the questions......

if I have two really low batteries, and the engine is started and the relays close, will there be a sudden rush of current

Yes, most definitely!!

, and will this sudden load on the engine charging system cook it?

Might, might not, depending on a few things, but mainly if or if not it has "fold back protection", if so, under(or near) max load, it'll most likely cycle it's output off and on to keep from cooking itself (over current will cause it to thermal protect itself)..... It will get quite hot at the very least

If yes, then I can ditch two of the relays and throw in some fat transistors and ramp in the connection,

Limiting current by making BIG silicon heaters would work, But...........

so maybe i would be fine with the relays?

Yes (SSR's would work too) but here's what I'd do FWIW, designed one or two things in my day, why not.....

If you're coding a microcontoller why not...............

1. logically switch in only 1 battery at a time, staggering the load current thus reducing inrush
2. monitor load current not only for the above switch over times BUT can also cycle the switching relay while the batt is coming up to prevent charge system over heating (several on the market sys do this, including ACR's)
3. parallel only after the two batts have individually and sufficiently both have high enough charge, say 80/90% SOC each)

Or buy one of these....Might have the above features and maybe even more? Might have to do some research or call the folks and ask.

Yandina Trollbridge36 Battery Combiner - TB36

The Yandina Trollbridge36® Combiner allows you to charge your 36V trolling motor battery from the 12V alternator on your main engine or a trailer hook-up. It works by automatically putting (3) 12V batteries in series when you need to run the trolling

www.defender.com