Batteries in a series-parallel configuration

[jasper79]

Hello All, I have a situation where a legacy design has the following characteristics:

- (6) 12V lead acid batteries in series with another (6) lead acid batteries
- This 24V system provides starting function for (2) 1000hp diesel engines
- Each engine is equipped with a 75A alternator
- The setup utilizes an isolator to keep the 2 alternators from seeing each other while charging
- The setup utilizes an equalizer for 12V loads

See the attached for current design and my attempt at a more balanced design.

I have found some literature on best practice for wiring the equalizer. And the isolator circuit is sound.

My concerns and questions:

Under current design, it looks like the first battery or 2 in each bank is getting the bulk of the charge but I am having trouble figuring how to analyze and get real values-
How to wire the batteries such that they are balanced during discharge and charge?
How to calculate charge going to each battery in current set-up and new set-up?

Thank you for your input!

 

[waross]

That sounds like overkill.
I was responsible for a pair of 2000 HP, 16 cyl diesels.
Each started with two 8D batteries in series, charged by 24 volt alternators.
Completely independent of each other.
Use a dedicated battery and a battery charger for the 12 Volt loads.
A failure in any system will not affect any other system.
Check with the diesel manufacturers as to the CCA required to start those engines.
Yes, I understand that your scope of work may not allow such changes.

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

 

[waross]

Under current design, it looks like the first battery or 2 in each bank is getting the bulk of the charge

Your jumpers should be sized for the starting current which may be over 1000 Amps.
Charging will start at 75 Amps and quickly drop to a low value as the batteries come up to charge.
There should be equal voltage across all batteries.
More of a concern to me will be the effects of a failing battery.
As a battery starts to fail, more of the current will be lost as heat and less will be used to charge.
As the failure progresses the failing battery may actually lower the rate of charge of the other batteries, when the alternator is not running.
With over 20 batteries in parallel, identifying and removing failing batteries was an ongoing issue.


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

 

[jasper79]

Thanks for the replies... Word is the overkill was to mitigate the effects of cold North Dakota winters on a starting system.

I have modeled the circuit in Multisim (National Instruments) but it is limited and won't let me see each battery - so I have to model it as one battery with 6 [internal] resistances in parallel.

The results were obvious... the battery nearest the alt is getting full current, the next 5/6 current, the next 4/6, and so on.

I would like to know if that is how the system is really behaving- I don't have a field unit to test the accuracy of those results.

 

[itsmoked]

1) You do not have, as stated in your original post, "6 batteries in series". Very confusing statement.

2) That is a mess. You should follow Bill's two 8Ds in series for each engine. Both separately charged by each engine.

3) If you need the ability to use the other engine's batteries to start an engine then maybe use some knife switches.

4) Yes. The lead lengths can cause battery charging issues, though with that setup I doubt it. This is because the batteries aren't being cycled frequently and have days and days to self balance.

5) Typically why batteries like that fail to do the job is because they go flat over the summer and when called on in the winter are simply discharged. The solution is to keep them on an intelligent float charger.

6) Batteries these days are cheaper and crappier than ever before. Having more is a distinct liability because a bad '1' kills all the rest. The more you have more likely you are to have a bad one killing the rest!

7) 6) Above also means you're likely to get a fire or explosion the day one cell in one of those batteries shorts out. You will get the other five batteries jamming 5,000 amps into that now 10 volt battery. If you're going to have more than two batteries in parallel you need fuses in every battery lead to disconnect that battery when it has a cell short. Hence using two 8Ds in series!

8) Make sure the float charger has a remote temperature sensor that is carefully situated to correctly detect the battery temperatures.

9) Consider a heated battery pad under the batteries to keep the batteries at a reasonable non-ND temperature. This also considerably helps battery life as it helps with electrolyte circulation.

10) Take the battery isolator out, place it on a ND fence post and shoot with a 45-70.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

While this won't model the system exactly it may show the futility of an exact model.
Divide the 75 Amps charging current by 6, or 12.5 Amps per battery.
Calculate the voltage drop in each set of jumpers.
1st set at 12.5 Amps,
2nd set at 25 Amps,
3rd set at 37.5 Amps, etc.
The voltage seen by the battery at the charging end will be very slightly higher than the other batteries.
It will charge a little faster.
As it charges, it will demand less current and more current will be available to charge the other batteries.
The next battery in line will charge to the point that it will demand less current and more current will be available to charge the other batteries.
This will continue down the line.
As the batteries approach full charge, the charging current will drop and the voltage drops across the jumpers will drop.

As for Keith's comments:
3) If you need the ability to use the other engine's batteries to start an engine then maybe use some knife switches.
I was going to suggest that.
5) Typically why batteries like that fail to do the job is because they go flat over the summer and when called on in the winter are simply discharged. The solution is to keep them on an intelligent float charger.
I agree with reservations. Not every battery charger is suitable for continuous charging of a battery.
I installed a number of sets with integral battery chargers in the control panel.
On the sets without a charger, when the battery failed, I got a call that the set wouldn't start.
On the sets with a charger, when the battery failed, I got a call that the set wouldn't start, and that the battery had exploded.
Be very careful selecting a charger.
Don't blindly trust the set manufacturer.
Had a program to load test the batteries once or twice a year.
6) Batteries these days are cheaper and crappier than ever before. Having more is a distinct liability because a bad '1' kills all the rest. The more you have more likely you are to have a bad one killing the rest!
I agree.
7) 6) Above also means you're likely to get a fire or explosion the day one cell in one of those batteries shorts out. You will get the other five batteries jamming 5,000 amps into that now 10 volt battery. If you're going to have more than two batteries in parallel you need fuses in every battery lead to disconnect that battery when it has a cell short. Hence using two 8Ds in series!
I agree. Note: The explosion that I have seen were with a single battery.
Multiple batteries in parallel multiplies the chances of an explosion and adds failure modes.
A steam explosion from boiling electrolyte may blow the ends out and or separate the battery.
When continuous charging boils the electrolyte below the top of the plates, and charging continues, you may experience a Brown's Gas explosion of the explosive hydrogen/oxygen mixture above the plates.
This explosion violently removes everything above the plates except the battery posts.

8) Make sure the float charger has a remote temperature sensor that is carefully situated to correctly detect the battery temperatures.
9) Consider a heated battery pad under the batteries to keep the batteries at a reasonable non-ND temperature. This also considerably helps battery life as it helps with electrolyte circulation.
I agree. Take care that the heat and the temperature sensor do not interact negatively.

10) Take the battery isolator out, place it on a ND fence post and shoot with a 45-70.
A 45-70 cartridge will cost me $2.10
A 223 cartridge will cost me $0.60
I don't have Keith's unlimited budget. grin

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

 

[waross]

With jumpers that would produce those results with 75 Amps charging current in real life, you would never be able to crank the engine.

Word is the overkill was to mitigate the effects of cold North Dakota winters on a starting system.

The Alberta Clipper is a sight colder before it leaves Alberta.
I have never seen such a mess up here in Alberta.
If it's too cold for the batteries to start the engine, it is too cold for the engine to be started.
There is more than one way to cope with starting diesel engines in cold weather.
If you can give us a hint as to the use of these sets we can suggest a tried and true method of cold weather starting that won't rape a cold engine.

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

 

[davidbeach]

First off - no cold engines. Block heaters and lube oil heaters.

 

[itsmoked]

Yes Bill, a charger that cut-off completely would be an excellent choice actually probably superior.
This could be done with a cheap-ass idiot style charger on a time clock. On for like an hour every week. Something like a 5A or 7A charger.

My thinking was that big 'ol lumbering 45-70 bullet would get the most attention from the idolotrator.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

First off - no cold engines. Block heaters and lube oil heaters.

I agree. I installed a set in the tropics that was factory set to refuse to crank if the coolant temp was below about +65F.
When I did the install I cheaped out and after Canadian winters thought;
"No need to connect the block heater in this climate."
One morning I got a panic call;
"The set won't start."
I was wearing my usual short sleeved tee shirt with no jacket when I connected the circulating coolant heater.
An interesting comment on the oil heater.
Normally the oil is cooled in the oil cooler by the engine coolant.
When the oil is cold but the block heater is working, the coolant is quite a bit hotter than the oil.
At start-up, an oil cooler, if fitted, will be heating the oil, rather than cooling it.
And cold weather starting;
The block heater option on a GM vehicle has a thermo-switch built into the cord end. The block heater in a GM or Chevy does not turn on at temperatures above 0F or -18C. (Including the diesel Duramax models.)

Back to the sets in question and battery chargers.
I would use two large batteries in series and 24 Volt alternators.
Exercise the sets regularly. You can alternate, #2 set one month, #2 set on alternate months.
If a set won't start or has trouble starting close the knife switch that Keith suggested.
Remedy the issue and then leave the switch open.
If there is no grid power available, there are circulating heaters that burn diesel fuel and run on 12 Volts that are used on a lot of heavy trucks in cold climates.
Another option is a small boiler plumbed into the cooling system.
I saw a system in the Yukon territory used by a major trucking company.
They installed a boiler and circulating pump in their shop.
At regular intervals on the outside of the shop wall were pairs of quick connect couplers.
When a truck was parked, the drive used a pair of hoses to connect his cooling system to the couplers and hot coolant was continually circulated through the engine.
Shifting a heavy transmission when the grease is at minus 30 is something else.

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