Alternator load - Case Study

[gpounder]

What it takes power wise to operate a class A diesel going down the road

Case study

Starter/chassis battery
10 amps - Engine/ignition systems
12 amps - Recharge battery rate after 15 min
16 amp - Lights (driving at night)
11 amps - Vent fan only (max)
23 amps - Dash air conditioner (max)
61 amps - Total Alternator load

House battery
1 amp - misc led switch lamps
.75 amp - water heater control board on propane
3.2 amps - Inverter powered on with Microwave in standby
7.2 amps - 18cu residential fridge running
4.3 - 15.3 amps - in motion high performance starlink
2.75 amps - radio and rear camera
137 amps - operating microwave
2.75 amps - TV only without dish box
14.9 - 30.2 total range without microwave running or TVs on.

Total load on Alternator going down the road with charged house batteries is 91.2 amps.

Observations:
Alternator load can be less by turning stuff off. It can be a lot more if you need to also charge the house batteries. Some of the loads are intermittent. A low lithium battery can add 50-150 amps if charged directly and that can overload a alternator without protection.

Recommendations:

1. Start your trip with fully charged batteries.
2. Preload fridge and let it cool fully before you leave to reduce run time.
3. Run the generator for the first hour or more to allow it to charge batteries and lighten the alternator load. Turn on the roof air conditioner to load up the generator.
4. Turn stuff off with the breaker so it will not draw any power even if on standby (example microwave and TVs)
5. Solar can help during the day. My 800 watt system will contribute 41 amps at peak.

Every rig is different and how you operate will be different. For example some people do not run the headlights during the day.

[twinboat]

If this is a common occurrence with many DP RVs , why the concern.
Let the alternator do its job.

These things must have 200 amp
alternators so 91 amps is less then 50 % output.

[GypsyR]

Or use an ammeter.
The way I look at it is that the alternator is doing none of that except keeping the batteries charged up and they are what's carrying the load.

[2cyber71]

I glad Winnebago out a 240 amp alternator in my Horizon 42Q.
Charges everything quick.

[gpounder]

Originally Posted by GypsyR

Or use an ammeter.
The way I look at it is that the alternator is doing none of that except keeping the batteries charged up and they are what's carrying the load.

I put a amp meter on the alternator line. This was the result and it is clear the batteries do NOT carry the load when the alternator is running. The only time the batteries would contribute power is when the alternator is unable to provide it.

[gpounder]

Originally Posted by twinboat

If this is a common occurrence with many DP RVs , why the concern.
Let the alternator do its job.

These things must have 200 amp
alternators so 91 amps is less then 50 % output.

My Country Coach has just a 145 or 165 amp alternator so I am concerned and will need to take my own advise and turn stuff off.

[vito.a]

Interesting post.
Back in 2004 our Monaco came with a Leece-Neville 270 amp alternator.

[chuckftboy]

Seems that 160-amp alternators are the norm now but still a lot of charging power.

[BayRunner]

I was in the engine bay just the other day looking at this. My Ventana came with a 240 amp alternator.

[RoadTrip2084]

Originally Posted by gpounder

I put a amp meter on the alternator line. This was the result and it is clear the batteries do NOT carry the load when the alternator is running. The only time the batteries would contribute power is when the alternator is unable to provide it.

Yep, I can monitor my LiFeP04 house battery and can see that it rarely contributes much while the alternator is running.

I have a 200A alternator, not something I worry about.

[Jrampey]

Hence, the reason I installed a 40A dc to dc converter between my 400Ah lithium batteries and the alternator. I also installed an A/B switch. A = 40A converter, B = straight thru to alternator. In position A - the batteries take 40A from the alternator for hours of driving…I’ve never left it in position B for long, but I have seen 90A+ at idle from a 140A alternator. That load for hours can’t be good.

[RoadTrip2084]

Originally Posted by Jrampey

Hence, the reason I installed a 40A dc to dc converter between my 400Ah lithium batteries and the alternator. I also installed an A/B switch. A = 40A converter, B = straight thru to alternator. In position A - the batteries take 40A from the alternator for hours of driving…I’ve never left it in position B for long, but I have seen 90A+ at idle from a 140A alternator. That load for hours can’t be good.

Agreed. My 60A DC-DC charger was only about $150USD, well worth it.

[twinboat]

The main benefit of a DC to DC charger is that it boosts alternator voltage output to lithium specs.

If your lithiums are charging at 60 amps, your alternator is sending it more then that .