Which batteries should start the generator?

Ok–fair warning up front: this post is probably going to sound like a little bit of a rant. 

One of the high-current 12V loads I needed to figure out how to power as part of the 48V lithium-ion project was the generator, and if you’ve been following the lithium house battery project, you already know that I resolved the issue, at least for now, by connecting the generator to the chassis batteries.

While it may seem like an obvious solution in my case, it remains that most motorhomes are configured with their generator powered by the house–not chassis–batteries.  I’ve never liked that configuration, and I thought I’d take a few minutes to discuss why.

To start, let’s ask the most obvious question–why would we need to start a generator?  That depends on what your motorhome is equipped with, but usually on the list is needing to charge the house batteries.  Those batteries are typically deep cycle batteries, made to maximize reserve capacity, not cranking amps.

Starting anything but the smallest of generators requires far more current than any of the other house systems are likely to require.  A weak house battery bank, whether from age or just the fact that it’s been used to the point that it needs recharged, is going to struggle during the cranking process.  With various electronic things running on 12V power (think refrigerator, furnace, and water heater control boards, radios, and possibly wireless equipment), you may see undesirable behavior, such as clocks needing reset.

Not having many cranking amps to begin with from the house batteries, and expecting to use them to start the generator when depleted can be a tall order.  On the other hand, if we use the engine starting batteries to start the generator, we’d be asking batteries to do something far easier than what they normally do every time you fire up the engine.  With no house systems powered by them, they should be very nearly fully charged when the house batteries are depleted to the point that we need to start the generator.

What are the other advantages to this configuration?

  • If you have a diode-type isolator (pretty common), running the converter (or inverter/charger) doesn’t charge the chassis batteries.  Connecting the generator to the chassis batteries gives you a method to charge them without running the RV’s main engine.
  • A problem on the house system doesn’t prevent you from being able to start the generator.
  • Lights won’t dim while the generator tries to start.

What disadvantages might there be?

  • Depending on the location of the generator and chassis batteries, the cable runs may end up a lot longer.
  • If there’s a problem with the generator drawing power when it’s not on, or if it isn’t producing 12V power to recharge the starting batteries, it’s possible to leave yourself stranded. (Though a battery separator failing closed would have had the same effect anyways)
  • Making the switch might require relocating the separator/isolator and/or running new battery cables.

Regardless of which batteries do the work, the generator takes a lot less to get started, so even if the chassis batteries aren’t fully charged, you’d still be able to start the generator to recharge both battery banks.

Having made the switch on my own RV, I can say that the generator definitely cranks much more quickly, and especially so in cold weather.  Conveniently, it was also the least complex way to deal with the conversion of the house battery bank to 48V.

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