Always been a question in my head and I always forget to ask anyone.

When the house batteries are low I can either charge them by running the generator or turning on the engine. Does it load the house batteries any faster if I run both the generator and the engine at the same time? Or can the batteries only charge at a given speed?

Thanks,
Christine

quote:

Or can the batteries only charge at a given speed?

You are correct. Batteries accept charge at an Amp rate determined by the combination of their size and State of Discharge.

As a generality, a modern [switching power supply] "hot" converter will supply your batteries with their maximum acceptance rate unless you have more than two house batteries. In this case also running the engine doesn't help.

OTOH, if your converter is old (especially a so-called Linear model-the ones that hum from a large internal transformer) your engine alternator could easily outperform it.

To give a useful answer folks here will need to know how many batteries of what size you have. Really need to know the brand and model of your converter also.

Lew

The batteries could be damaged if you regularly apply a charge to them higher than the manufacturers' recommendaed charge acceptance rate, for long periods.

AGM batteries have very high charge acceptance rates compared to flooded cell lead acid batteries.

Some batteries may tolerate a vey high rate for a short duration during the "bulk" phase of charging, then drop back to a lower charge rate for the longer absorb" phase of charging.

Periodically flooded lead acid batteries, (and some AGM batteries), require equalization with charge voltages and currents much higher than that used for normal charge cycle.

IMO, to answer the posters original question, you need to know the max amount of charge the converter is capable of putting out when running the genset. If the battery bank is pretty low, and the converter is essentially a small battery charger with limited output, then running the MH engine too would bring the battery bank up quicker than the genset alone. Seems to me, tho, it'd would have to be an unusual situation to run both due to the fuel useage.

Batteries only charge at a given speed but... That given speed is controlled by the voltage applied to the battery and that is limited by the charger output current.

Just rough examples but if you put a 14.8 volt charger on your low battery and it is able to put out 40 amps the battery will pull that 14.8 volts down to a lower voltage and accept the entire 40 amps that the charger can put out. As the battery charges it will rise in voltage but the current will stay at maximum until at some point the battery voltage will rise to the 14.8 volt level and the current will taper off.

So if your alternator and converter are putting out voltages high enough to charge the battery their currents will add and the battery will charge faster initially.

Now neither the converter or alternator are likely to be set up for charging the battery, they are usually set for longest battery life with some level of charging (or on awful converters a trickle charge) since most folks don't want to pay for a good charger.

So what happens is they add their currents until the battery voltage gets to their output voltage and then they stop charging. So you'd do well to know when each quits and turn off the engine/generator on it at that point to keep from wasting fuel.

That doesn't give you a good charge and the batteries aren't full but it isn't practical to fully charge your batteries from an engine on a daily basis. What batteries want is a two stage charge, first stage is bulk charging that gets the battery mostly full at a high current, the second stage is at a lower current that slowly tops off the battery. Getting the last few percent into the battery can use twice the fuel getting it up to that point does.

However you should fully charge your batteries every two to three days to reduce sulphation and extend their lifetime. At $4.00 per gallon and $70.00 per battery how often and how much becomes a balancing act.

One reason folks like solar is that they can charge the first stage using their generator and then let the solar take over the last stage where its lower current and zero fuel use really shine.