Loading one Battery in a Parallel Circuit

[172heavy]

I have what may seem like a dumb questions but I am going to ask anyway. If I have a parallel circuit with two 12v batteries. So positive and negative are connected on one side while the positive and negaive are delivering 24v from the remaining two terminals. If I install an item that draws 40 mA directly to one battery (12v) will this cause a problem if so what and why?

 

[itsmoked]

First off those batteries are NOT parallel they are hooked in series.

Secondly, you shouldn't have a problem with a 40mA load hooked to one of those batteries. I would suggest you hook it across the lower battery. That's the one that has the cable running off to somewhere from it's negative terminal.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

What size are the batteries?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The way I picture it, you have two 12V batteries in series to get 24V. You also have one load across only one of the batteries, getting 40mA and 12V. Do you have any 24V loads? If not, you effectively have one 12V battery loaded and one 12V battery unloaded.

 

[rcw retired EE]

The problem is unequal currents on both charge and discharge may affect battery life. During charge, both batteries obviously get teh same current but the one battery doesn't need that many amps and can get overcharged before the other one gets close to a full charge.

I did this on a cross-counrty bus with a large 24V battery. We needed 12V to run the 1 kW inverter for the team's X-box, TV's and refrigerators. The inverter amp draw was more than I liked on the one battery so we rigged some voltmeters to monitor each battery's voltage and had a plug arrangement to swap the inverter between batteries safely every few hours. The bus engine alternator/charger could only sense and charge 24V so we had to watch the voltages and electrolyte levels to avoid overcharging one battery. It worked well enough to complete the 3 week trip.

In an old aluminum plant one switchgear lineup was 250VDC control and the newer one was 125 V. 20 years later, the two lead calcium battery banks were replaced with a single bank with a center tap for the 125V loads. Each half of the bank had it's own 125V charger. That worked well for another 15 years.

 

[itsmoked]

That would certainly be the case with a big load. But a typical large deep cycle battery has about 40mA of self discharge going on anyway. So a little more than that probably isn't going to upset the apple cart. Flooded batteries will also re-balance themselves unlike most others.

That is IF that's what these are..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[172heavy]

Yes you are correct "it's smoked" they are in series. As far as size they are 8d 160ah equipment batteries. We installed an hour meter which transmits hours every hour for about 3 seconds. The equipment is mostly wired like I described for 24v from 2 12v batteries. When the equipment is running there are 24v loads for starting etc. the problem is the batteries are going dead very quickly approx 3 days. The hour meter is hooked up outside the battery isolator so when the equipment is idle the hour meter is the only item that is hooked up and in is hooked up on one battery only. I believe this uneven loading is the problem. We normally hook the hour meters up to 24v just like everything else on the equipment and we never have an issue with discharged batteries. I measured the current draw and it is approx 40mA. at peak during the transmission of data the draw is 500mA.

 

[172heavy]

I wanted to clarify something from the post above. When I say idle I mean parked for a few days. Also we had some guys who installed this system for reason only hook the hour meters up to 1 12v battery.

 

[ccjersey]

Hardly seems likely to be the cause of the dead batteries. Any chance it is sending a lot more often for some reason? One thing you could do is test the state of charge of the two batteries next time they are dead. Of course if it's only one dead, it isn't likely it will even attempt to start the engine. If it happens to be the battery without the hour meter, that would answer your question. Should also be easy to swap the hour meter from one battery to the other and see if the dead battery follows the hour meter.

You mentioned a battery isolator. Is this a manual disconnect? How sure are you that this is actually being used?

Are 24 volt transmitting hour meters available? Any chance of disconnecting it for a planned idle period as a test? Sometimes batteries self discharge at a greater than expected rate. In my experience, heavy equipment batteries do not usually give long service life. You may get better service from another brand.

 

[waross]

40 mA or even 500 mA on an 8D? The hour meter is not your problem.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

What waross said. Something else is draining your batteries dead. Please check your wires for some grounds.

 

[waross]

I would expect an 8 "D" to last over three weeks with a 500 mA load.
Further, the hour meter should be powered only when the equipment is running and charging.
If you want to leave an hour meter connected to a battery 24/7, you may do better with a calender.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Excellent Point Bill.

I'd disconnect the fully charged batteries and insert your DMM set to DC amps. This is with nothing turned on of course. That will show you how much, if any, drain normally, or abnormally, is occurring. If you see something you can easily track it down with the ammeter in place.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

How old are the batteries? Have they been disturbed recently? Older batteries tend to accumulate a layer of debris in the bottom of the case. I have seen older, heavy batteries such as 8 "D"s removed and then replaced. If they contact the floor or battery box when they are tilted, the debris may reposition along one side so as to contact the bottom corner of the plates. The batteries then do not hold a charge well.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

waross, that would explain the often heard consideration about leaving lead acid batteries on concrete kills them.

I'd never thought that placing the battery on concrete would damage it, but your explanation makes a lot more sense, and also explains the inference of concrete damaging batteries.

 

[waross]

It's the most logical explanation that I can offer for the urban legend that concrete kills batteries.
We moved a couple of 8 Ds that were working well. They are heavy and awkward and had to be tipped to remove them from their normal positions and again to replace them. They were not dropped, but there were a couple of jolts against hard surfaces. From that day the two batteries would not hold a charge. If you have ever taken an old car battery apart you will be familiar with the layer of debris from the plates that accumulates in the bottom of the case.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[172heavy]

I tracked down the parasitic draw on a few of the machines that were in question. One was an FM radio another was a CB radio that were drawing current.

As far as the hour meter being energized while the machine is off the hour meter part is not on just the radio which is listening for the data accumulator which is mounted on a roving vehicle. These devices are set to transmit once per hour then they sleep. Which draws approx 40mA.

Question, If an 8d is rated at 255 ah and you have two in series you should get 510ah of capacity correct? So if you have a draw of approx .340 amps or (340mA) that should give an approximate 100% discharge time of 62.5 days ((510/.340)/24).

Part 2 if the above is correct in order to discharge 20% or use approx 102 ah of capacity it should take approx 12.5 days ((102/.340)/24).

Does this reduction in capacity translate directly to a reduction in CCA? If a vehicle requires 800 CCA to start and you reduce the AH of a Battery System capable of producing 1000 CCA by 25% would you be left with approx 750 CCA of cranking power? Thus not being able to start the vehicle? I am just trying to fully understand the relationship between the AH and CCA.

Thanks

 

[sibeen]

Question, If an 8d is rated at 255 ah and you have two in series you should get 510ah of capacity correct?


No, it remains a 225 A/h system, albeit at a higher voltage. if you wish the higher A/h rating then the batteries need to be placed in parallel.

 

[itsmoked]

There is no direct relationship between AH and CCA other than the two often get bigger with bigger sized batteries.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[burnt2x]

There are formulas I know of but I don't really use them. I googled and found out in one Hawaii University site where Reserve Capacity was approximated equal to CCA divided by 5. It further mentioned an approximation of AH from RC as AH = RC/2 + 16. E.g, a 1000 CCA is equivalent to RC = 1000/5 = 200 minutes. And the, the approximation of AH yields 200/2 + 16 = 116 ampere-hours.

(Please see that when you divide 116 AH by 5, we get 116/5 = 23.2 amps, which is very close to the defined rate of discharge in RC, i.e. the number of minutes the battery is delivering 25 amps until the terminal voltage drops to 10.5V.)

I know that once, battery manufactures rate batteries on a 5-hour validation test period. Later, manufactures used 1-hour basis for simplicity in marketing. Other ratings basis are the 8-hour, 10-hour and 20-hour.

Since CCA is the amount of current that can be drawn from batteries (at 0 - minus 18 deg F) for 30 seconds before the battery voltage drops to 10.5 volts (for 12-volt batteries), dividing the approximated RC by 2 (two, thirty seconds in one minute) and adding a little ampere-hours to account for the non-linearity will give AH values very near the rated AH. Other automotive battery manufacturers just approximate AH = RC x 0.6. (AH = 200 x 0.6 = 114, in the example).

Still, I don't use these formulas and just brought these up here for people to comment.