Running MH engine for air con etc.

msg · Guru

Running MH engine for air con etc.

PeterD · Guru

You will need quite a few hours to fully charge your batteries (even one battery, more batteries will take longer.)

Peter_n_Margaret · Guru

If the house batteries are close to the engine and/or you have very heavy cables you will be able to get quite high charge rates from the alternator PROVIDED it is not a "smart" alternator.
As the battery comes up in charge, the rate will decrease, but by then you will have dumped a lot of energy into the battery quite quickly.
I do this and the initial charge rate is around 70A from a 85A alternator (which is in addition to anything being produced by the solar panels). The charge rate will also vary depending on the batteries you are using.
Running the engine for an hour or so will get you out of trouble for quite a while. You may need to run the engine at a higher than normal idle speed (maybe 1200rpm?) depending on the alternator and pulley diameters.
There is very little risk of any engine damage. Semis rarely turn their engines off for a meal stop and most tour busses do it all the time to keep the aircon running during stops.

Cheers,
Peter

msg · Guru

Thanks Peter, yes the batteries charge nicely over the hour, maybe not fully, but enough to last the night. The hot water heater is a heat exchange or 240v so that takes until the engine gets warm. Nice to know I'm not killing the engine. Also, the furry friend is a bit heat sensitive and it helps her if she can get cool for a bit also. I have heard differing views, but think I might risk it in emergencies more often.

Peter_n_Margaret · Guru

I think the message for looking after the engine is NOT to do this for 10-15 minutes on a regular basis, rather get the engine up to temperature and run it for an hour or so on a less frequent basis.

Cheers,
Peter

T1 Terry · Guru

The important things to avoid engine damage are not to slow idle for long periods, let the engine reach full operating temp each run and add a bit of load for the engine to work on to avoid glazing the bore. This can be achieved via alternator load and air conditioner load. If it has a smart charging system then you will need a DC to DC charger as the smart charging system uses a max voltage too low to put a decent charge into a house battery.
Depending on how low the house battery is and the capacity as to how long it can accept a high rate charge. The higher the voltage the longer it will accept a high charge rate before the battery voltage reaches the same voltage as the charging voltage, then flow stops. The closer the house battery voltage is to the charging voltage the slower it will charge. A DC to DC charger may restrict the current flow between a very discharged battery and a high voltage alternator, but a 40 amp DC to DC charger will provide 40 amps at up to 14.4v for as long as the battery will accept it. This means you know that a 1 hr engine run will put 40 Ah into a battery discharged to 50%, so back to 90% roughly for a 100Ah battery but only 70% for a 200Ah battery. Cetek produce a unit called a smart-pass that will allow up to 80 amps to bypass the DC to DC charger but will not allow the charge to flow back from the house battery to the start battery once the DC to DC charger gets the house battery voltage higher than the start battery voltage. there are a number of these smart pass type units available so it might be worth doing a bit of research on the web to find one that suits just what you want it to do.

T1 Terry

msg · Guru

Thank you Terry I will check it out.

Santa · Guru

I have done this on the odd occasion to boost the batteries, not something I would do regularly.

-- Edited by Santa on Friday 9th of February 2018 09:57:29 AM