How to check that the battery is getting the correct charging voltage

[Rod1883]

I'm getting a bit of acid bubbling out of the battery cell caps on our MG Midget. The set up is pretty similar to the TR2 - positive earth, a dynamo and the same control box/voltage regulator - part number 3H1835.

It is possible I've slightly overfilled the cells and that is the problem, but I'd like to check that the battery isn't being overcharged in case that is the cause. I believe all I need to do is put a voltmeter onto the battery and check the reading when the engine is running and charging. What voltage levels should I be looking for?

I ought to check the TR voltages too as I'm not sure the battery charges very well...

Thanks

Edited June 3, 2021 by Rod1883

[Lebro]

No higher than 15V, & preferably not above 14.5V

Bob.

[Rod1883]

Thanks Bob. Is there also a lower figure that needs to be achieved to ensure charging?

[RobH]

The electrolyte should just cover the plates - no more. 

When on charge with the engine running, the voltage will be somewhere between about 13.9 and 14.7 volts depending on adjustment  of the dynamo regulator, but it doesn't tell you anything about the state of charge of the battery. 

To check the battery you need to measure its open-circuit voltage when not on charge, after standing for some time.  A fully charged battery will read about 12.6 to 12.8 volts depending on the temperature. 

Typical curves are here:

https://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

Note that these are for a plain lead-acid battery not a 'calcium' type. 

The other way of testing for charge is to use a hygrometer to test the specific gravity of the electrolyte as described in the attachment.  

 

Edited June 3, 2021 by RobH

[Rod1883]

Thanks Rob.

Yes - just covering the top of the plates was what I was always taught and understood. The difficulty with the battery on the midget is that the plates aren't visible - they have a plastic cover with a side gap to allow the electrolyte to be topped up. I couldn't see the level so topped up to just above this plate and so it is very likely that I have caused my problem doing this and I just need to remove electrolyte until the cover is just uncovered - not a good design.

[John McCormack]

6 hours ago, Lebro said:

No higher than 15V, & preferably not above 14.5V

Bob.

I understood that a generator will charge at higher than 15V, maybe as high as 16.5V, with a flat battery or just after start. It should then settle down to about 15V.

As repro regulators aren't adjustable like the originals I am happy if it is charging over 14V.

 

[Ralph Whitaker]

8 hours ago, Rod1883 said:

I'm getting a bit of acid bubbling out of the battery cell caps on our MG Midget. The set up is pretty similar to the TR2 - positive earth, a dynamo and the same control box/voltage regulator - part number 3H1835.

It is possible I've slightly overfilled the cells and that is the problem, but I'd like to check that the battery isn't being overcharged in case that is the cause. I believe all I need to do is put a voltmeter onto the battery and check the reading when the engine is running and charging. What voltage levels should I be looking for?

I ought to check the TR voltages too as I'm not sure the battery charges very well...

Thanks

I would suspect the regulator box. There are 2 coils in there that control the output from the dynamo and either could be sticking. As the battery charges up the control box should reduce the amount of current being passed to the battery to prevent overcharging and boiling of the electrolyte. If the voltage from the control box exceeds the battery voltage it will keep charging. They are quite cheap so I would be putting another on before the battery is damaged, or borrow the TR one to see if it makes a difference.  Some "gassing" is normal though, and it does sound as if you have overfilled it. I would have thought though that if you cannot see the plates it sound like a sealed for life battery that should not require any topping up, or it would have a self regulating system so that you cannot overfill from the side hole.

Ralph.

 

[Lebro]

3 hours ago, John McCormack said:

I understood that a generator will charge at higher than 15V, maybe as high as 16.5V, with a flat battery or just after start. It should then settle down to about 15V.

As repro regulators aren't adjustable like the originals I am happy if it is charging over 14V.

 

15 Volts is the maximum the voltage regulator should be set to regardless of the battery's state of charge. If charging at a high current (due to battery being in a discharged state) then the voltage will be lower than 15 due to the current limiting design of the 2 coil regulator.  Assuming the voltages were ok in the past, then it would be worthwhile cleaning the contact points on the regulator coil (the smaller of the two sets of points) just give them a polish with some fine abrasive paper, then blow away any remaining grit.

Bob.

[Rod1883]

Thanks Ralph.

When I say I can't see the top of the plates - it is the way the filling hole is designed. The battery isn't the sealed for life type, there are the usual screw type cell caps to remove for filling. The issue is that in each cell is a plastic covering over the plates with a side gap for filling - makes it difficult to see the electrolyte level until it seems it is filled too high......

 

..and thanks again Bob. Will check that. I also have a spare control box that I can check and try.

Edited June 4, 2021 by Rod1883

[John McCormack]

11 minutes ago, Lebro said:

15 Volts is the maximum the voltage regulator should be set to regardless of the battery's state of charge. If charging at a high current (due to battery being in a discharged state) then the voltage will be lower than 15 due to the current limiting design of the 2 coil regulator.  Assuming the voltages were ok in the past, then it would be worthwhile cleaning the contact points on the regulator coil (the smaller of the two sets of points) just give them a polish with some fine abrasive paper, then blow away any remaining grit.

Bob.

Thanks Bob. I'll do a check on the TR2s in the morning.

[RobH]

3 hours ago, John McCormack said:

I understood that a generator will charge at higher than 15V, maybe as high as 16.5V, with a flat battery or just after start. It should then settle down to about 15V.

 For interest, the controller adjustment process is here:

https://mgaguru.com/mgtech/books/pdf/Lucas_Generator_and_Control_Box_Tests.pdf

 

[Lebro]

I note that the voltages specified in the above sheet are with battery disconnected. 

I prefer to set with battery fully charged (from external charger) & connected to the car.

A moving coil meter is the best sort to use for this measurement, as it will not respond to transient voltage variations.

Bob.

 

[RobH]

This is one of the cases where an ammeter is a boon and shows things a voltmeter cannot. It will be obvious if there is a continual charge going into the battery.