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Estimating alternator capacity

December 07, 2015, 09:26:50 pm

For those forum members who are considering (or in the process of) upgrading their OEM 12V systems (you know who you are... )

Here is an interesting video I ran across, showing how to estimate alternator requirements with a clamp on current meter. A school bus may not be "exactly" the same as a coach, but I thought it was comparable, as a lot of us have added accessories above and beyond the stock coach setup. The key thing is to consider loads and situations that you might not think of, i.e. driving at night with headlights on high, fog lights on, defrost fan(s) on high, wipers on high, the fridge running on the inverter, DW making coffee, kids watching TV, etc. The 3 main categories of alternator amperage draws are:

- Amperage to run multiple accessories all running concurrently (headlights, running lights, fog lights, fans, wipers, fridge, coolant pump, etc)
- Amperage to run accessories that could be used (coffee maker, microwave, TV, 12V air compressor, etc)
- Amperage just to run the coach (engine and transmission electronics, dash gauges, Silverleaf and Audit displays, rear camera, etc)

Also, the recommendation is to take your estimated amperage load measured above and add 20% (multiply by 1.2).

Estimating alternator capacity

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Re: Estimating alternator capacity

Reply #1 – December 08, 2015, 04:07:23 pm

Yes but...  This applies to school buses that have a pretty fixed service cycle and does not differentiate between intermittent loads - wheel chair lift and warning lights for example - and continuous loads such as headlights.  It may apply to a motor-home that has heavy charging loads when traveling IE charging house batteries from the alternator.  Each application is different and each duty cycle is too.  Take a semi running over the road.  Once the starter draw is made up the total demand on the alternator is quite small especially in the daytime if no lights are in use.  At the other end an ambulance or fire appliance may sit with the engine at idle or with the fast idle engaged with a lot of lights and systems running for an extended period.  In this case an alternator with a high output at low engine revs is required.  In this case it must supply all the demand without calling for make-up from the battery.  Each case is different so you must run your own calculations per the video to size your charging system.

There is a little more to this but the video is a good start.

Keith

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Re: Estimating alternator capacity

Reply #2 – December 08, 2015, 05:21:11 pm

Keith,
Excellent comments, yes, this video is just an interesting exercise and topic for discussion.  Everyone needs to access their situation and "do what makes them happy"... I like to have some "reserve capacity", and enjoy tinkering... My 12v system (alternator /isolator /wiring) was mangled pretty badly when I got our coach, so it's been fun to make some improvements...

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