Battery Connections

[Bendit]

Just had an interesting experience which may be useful to fellow boaters.
After overnighting on the anchor, during our Christmas cruise (NZ) we awoke to find the house batteries reading 22V (24V system). This reading really means fully discharged and certainly the fresh-water pump and toilet operated as though the batteries were virtually dead. This happened on two non-consecutive occasions.

The weird thing was that the SOC meter was reading 89%, so I immediately figured that there must be a bad cell and an internal discharge, which the SOC would not see. Once restored via the DC genset, the voltage remained where it should at around 26V.

The second time this happened I did some math and figured that if one bank of two 12 V batteries in series, was paralleled with one only 12V battery, the result would be 22V. So dived into the battery box and yep, found a loose clamp on one terminal. Tightened it up and all is well.

Thought I was up for at least one new battery (group 31 Gel and expensive).

 

[DavidM]

Hmm! I don't follow your math. Maybe more importantly assuming that you have two pairs of 12V batteries in series/parallel, how could a loose connection result in one 12V battery in parallel with a 24V pair. Seems like it would be a zero volt battery in parallel with a 24V pair. A bad connection will measure zero volts at the output terminals.


David

 

[FlyWright]

Good find! One indispensable tool on my boat is a quality digital voltmeter. If the boat's voltmeter doesn't look right, a measurement at the batteries is a good double check.

Saw a similar issue with a friend's boat with new batts that were recently installed by a professional. The charger thought the batts were topped off, but the boat thought one bank was very low. It turned out to be a loose ground terminal connection on that bank. Easy solution!

 

[Capt.Bill11]

Hmm! I don't follow your math. Maybe more importantly assuming that you have two pairs of 12V batteries in series/parallel, how could a loose connection result in one 12V battery in parallel with a 24V pair. Seems like it would be a zero volt battery in parallel with a 24V pair. A bad connection will measure zero volts at the output terminals.


David

What he said.

 

[Bendit]

I will provide a diagram which shows 4 points where a disconnection can result in the situation described but it will have to wait until next week - we're out on the Gulf.

Math, which may not be correct but at least brought me to the solution:

The theoretical average of a 24v bank in parallel with 12v is 18v. However, the 24v bank has twice the capacity of the single 12v and thus will skew the voltage by 2/3 of the difference between 18v and 24v, that is 4v. That's how I got to 22v which was exactly what I observed.

 

[Peter B]

But if there was a dud connection between two 12v batts in series, as one side of a paralleled 24 v house set-up, wouldn't that dud connection mean no current or voltage from that half of the house bank..? Theoretically you wouldn't even get 12v from that half of the bank, although, yes, once the reconnection was made, you'd be back fully on song with your 24..? Like djmarchand and Capt Bill, I'm confused. Surely you can't have a series connection between two batts that's passing just 'some' current..?

 

[DavidM]

The theoretical average of a 24v bank in parallel with 12v is 18v. However, the 24v bank has twice the capacity of the single 12v and thus will skew the voltage by 2/3 of the difference between 18v and 24v, that is 4v. That's how I got to 22v which was exactly what I observed.

Nope, electricity doesn't work that way.

A 24V battery in parallel with a 12V battery will act like a battery charger to the paralleled 12V battery and will quickly boil (well actually disassociate) the water out of the 12V battery just like a battery charger set on equalization.

The voltage from the 24V battery will drop due to the high current supplied to charge the paralleled 12V battery. How far it drops depends on the size of the batteries and how long they are hooked up this way, but not by the physics/math you indicate.

So, it is possible for a paralleled 12V battery to drop a 24V battery to 22V, but I don't see how that can happen with a bad connection.

David

 

[bayview]

+1, a 12V battery across a 24V source will look like a short circuit to the 24V set and cause exciting results but not 22V.

 

[sunchaser]

Or he saw 22 V as a number that in reality is not relevant. The important thing is he has power. A photo of the wiring setup would be helpful.