Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 10, 2025 ...

Smart charging for the Engine start battery

June 22, 2017, 12:03:01 am

Lots of talk about optimal multi stage charging to maintain the the house batteries lifespan. How are the Engine start batteries treated by the alternator system?

Quote Selected

Re: Smart charging for the Engine start battery

Reply #1 – June 22, 2017, 06:57:40 am

You are asking a general question about a topic that has many variables:

Alternator specifications
Voltage sense line location
Battery specifications: Flooded, open or sealed. AGM. Gel, etc... (Don't get me started on lithium).
Isolator type: Diode vs ?
Wire size and length
Coach usage pattern
Temperature compensation system

Most alternators are dumb chargers, and only have voltage regulators to deliver a constant voltage. To maximize flooded lead-acid battery longevity, they must be at 100% charge, so they need a computerized four-stage, temperature-compensated smart charger capable of:

- Bulk
- Absorb
- Float
- Desulphate every second. This would be the Battery Minder/Tender type, with quick 16 volt pulses. This is not good for engine electronics, so turn off your engine battery switch while using pulse-type desulfators.
  OR
- Desulphate every week. My IOTA DLS-90 power supply/charger with IQ plug-in desulphates every week with 14.6 volts for one hour. This is not harmful to engine electronics.

As dumb luck would have it, most batteries are treated pretty well because:

A. Some alternators do a pretty nice job by blindly:
- Overcharging the battery. Not a problem if the water level is monitored, or,
  in the case of sealed batteries, the hydrogen gas recombines with oxygen gas to create water. Hence the acronym VRLA (Valve Regulated Lead-Acid) where, as long as not too much pressure builds up, the cells do not "gas" and water is conserved by recombination.
- Lowering the voltage and current after a time. AKA: Semi-smart alternator regulator design
B. Vibrations while the coach is moving act to desulphate the battery by stirring the electrolyte.

Tim - AKA "Mr. Battery"

Quote Selected

Re: Smart charging for the Engine start battery

Reply #2 – June 22, 2017, 09:06:05 am

Marine industry has multi state chargers available.  Link 2000r used to be available that had multi stage charging.

As Brett Wolfe posted here long ago I set my rebuilt 160 amp alternator at 13.6 volts. 

It's 1/5C charging capacity won't damage either the engines red top AGM's or the houses 8g8d's.

Funny enough all this was oem.

Float voltage for the house gels is 13.6 nominally.

I do have pulse desulphurators on the engine red tops.

My guru took them offf my house gels.  Said different chemistry

As a fully charged battery is 12.75 or so any charging over that seems ok?

The biggest help is the 500 amp seal land combiner. 

Dash plug in digital gauges show both batteries at 13.4-13.5 constantly on the magnum charger and our small solar which is also set at 13.6 volts at the battery.

We are medium power users with some refer compressor use on,our 7832 Dometic and our chest refer in the bays is normally on.

Between them we probably are using a similar amount of power to a residential refer but the power consumption is optional if needed.

More solar would lessen or eliminate most gen run time as most have noted.

More CB's.  Again.  But we like using our coach so you get to pay to play as usual

Quote Selected