Start Battery Problem

[wagwar]

On my 1981 MCI MC9, two of my start batteries (4 type 31 in series/parallel for a 24 VDC bank) are dead. It is the two with the 12 volt center tap for the headlights. I thought there was some kind of equalizer in the system - as I have seen mention of Vanner EQ's to balance the loads on the start batteries. However, I've never seen an equalizer or any other type of 'black box' in my bus.

So, I don't think there is one in my headlight circuit and a careful review of the 12 volt headlight diagram in the manual makes no mention of any kind of equalizer. To the best of my understanding, there is no provision in the wiring schematic for equalzing the start batteries.

Due to these dead center tap batts, I don't think I have any kind of equalizer. Am I supposed to have one? If so, where would I find it?  What does it look like? If I'm supposed to have one, why is there no mention of it in the manual - anywhere?  Is this an aftermarket item I need to add?  I've had problems with my start batts getting low and now I think it is because there is, in fact, no equalizing going on.

Thanks in advance.

[bevans6]

A Vanner is roughly 14" tall, 8" wide and 1 1/2" deep with three 1/4" post connectors on top.  Mine was mounted right beside the main disconnect switch on the forward wall of the front luggage bay.  With a center tap 12v headlight system there is no need for a battery equalizer because the 12v loads are themselves equal.  One pair of headlights operates from one battery (gnd to plus 12v), the other pair operates from the second battery (plus 12v to plus 24v).  The pair that is operating from the "top" battery is running on 12 volts from 12v to 24v, not from ground, is the only twist to the picture.  But the batteries see equal loads.  All this goes out the window if your headlights are just run from 12 volts to ground for both pairs of headlights, but MCI did have a stock system that used split pairs as I described.  

If your headlights are all ground to plus 12v, then you do indeed have a big imbalance that needs to be rectified with a battery equalizer.  If you have the typical 55W/60W headlights, then you have a possible load of around 25 amps if the high beams are on, which is around half the rated load of a typical Vanner equalizer as seen in bus use.

Now, if you have four batteries but only two are being used to run the headlights, as soon as you turn on the headlights you will create a charging imbalance between the two that see no headlight load and the two that are loaded.  That is incorrect, for sure.  You should probably tap both pairs of batteries for 12v use.  Also, the MCI does normally provide a 12 volt tap for other 12 volt loads on pin 53 (I think) in the main electrical panel beside the driver.  You might look to see if other 12 volt loads are present beyond just the headlights.

Brian

[wagwar]

One pair of headlights operates from one battery (gnd to plus 12v), the other pair operates from the second battery (plus 12v to plus 24v).  The pair that is operating from the "top" battery is running on 12 volts from 12v to 24v, not from ground, is the only twist to the picture.  But the batteries see equal loads.

I don't have a Vanner anywhere in my bus. I have a battery equalizer for my house 12v loads on the house bank, but not on the starter bank. In this situation, would I find two wires running from the start batteries to the headlight system? I am sure that there is only one wire running from my start battery compartment for 12v loads.

Now, if you have four batteries but only two are being used to run the headlights, as soon as you turn on the headlights you will create a charging imbalance between the two that see no headlight load and the two that are loaded.  That is incorrect, for sure.  You should probably tap both pairs of batteries for 12v use.

I believe this is my problem - one pair supports the headlights and the other pair is untapped.  How would I tap both pairs?  Would I need another wire for the 'other' headlight? I don't think my bus is wired that way, but perhaps I misunderstand how to tap both pairs.

TIA

[Rick 74 MC-8]

I just run my headlights off my 12v house bank. I have a piggyback 12 volt alternator to charge them when running down the road. I Used a 24 volt relay off the original switch to power them

Rick 74MC-8

[wagwar]

Hi Brian,

Do you have a wiring diagram for the balanced load method? I've studied my wiring diagram in the MC9 book and I only see one 12v center tap and one lead out of the start battery compartment and no equalizer. So, I'm not sure how it was supposed to work. I had my headlights fail once after we had been driving for several hours at night, I had that 'fixed', but we don't drive at night anymore. Still, I usually run with the headlights on during the day - thus the dead center tap battery pair. Do you think maybe I had just drained the 12v center tap battery?

If I add an equalizer, what size do I need and do you know of good model?

Thanks,
Jim

[buswarrior]

If you are familiar with how a residential 120/240 volt system works, you wire your 12 volt headlights in your 24 volt coach in a similar fashion, one on each hot leg in order to balance the loads.

Someone has a slick schematic of how to do it with a few relays, somewhat simpler than the MCI stock set-up.

The goal is the two headlights wired in series across the 24 volts, with a "neutral" or 12 volt centre tap connected in the line between the two headlights.

There will only be current in the 12 volt centre tap if you burn out a headlight.

Vanner is the go-to vendor for equalizers. Lots of good used take outs from the transit coach scrappers.

An equalizer simplifies your wiring and relay needs, and also gives you the 12 volt power for your other accessories, which are just as capable of ruining the batteries, just more slowly.

happy coaching!
buswarrior