DC circuit parallel vs series

[Weaver Road]

I am looking at battery sizing for an alarm system and seem to have a problem getting clear in my head what happens to the amperage of a set of batteries when in series. If the batteries are 12v and 7Ah, then I'm sure the voltage is 24v, but did the amperage go to 14?

 

[mdshunk]

Parallel two, voltage stays the same and amp/hours doubles.
Series two, voltage doubles and amp/hours stays the same.

 

[Lxnxjxhx]

I think

I think

. . .if you parallel batteries, you should have a diode in series with each, so that one battery doesn't ever charge the other (the charging current is unpredictable in that case). You'll lose 1v of battery voltage, less with Schottky diodes.

More stuff: buses use two 12v batteries in series, with some bus stuff running on 12v. Charging is with batteries in series, discharge is more complex. They need a special gadget to insure even charging.

Are they nicads? Nickel-metal-hydrides?

 

[mdshunk]

.More stuff: buses use two 12v batteries in series, with some bus stuff running on 12v. Charging is with batteries in series, discharge is more complex. They need a special gadget to insure even charging.

Some of my trucks have two batteries. They charge through a "battery isolator", which is basically two big-mamma diodes in one handy package.

 

[dereckbc]

. . .if you parallel batteries, you should have a diode in series with each, so that one battery doesn't ever charge the other (the charging current is unpredictable in that case). You'll lose 1v of battery voltage, less with Schottky diodes.

Huh? If you put a diode in the circuit, depending on polarity, will mean the battery can only discharge or charge, but not both. What am I missing in your statement?

 

[Lxnxjxhx]

diodes

diodes

Posting a schematic is a hassle, so here are words.
The load, looking back at the battery, sees the cathodes of two diodes. The battery positive posts see the anodes.

The schematic for charging batteries in parallel may be pretty messy. Also, depending on the technology, you may want constant current/voltage charging with a voltage/temperature cutoff. More diodes.

Marc, can I get a make & model on that isolator? I dealt with one, once, for school bus batteries.

 

[dereckbc]

The schematic for charging batteries in parallel may be pretty messy. Also, depending on the technology, you may want constant current/voltage charging with a voltage/temperature cutoff. More diodes.

I think this is being over thought. I have been in the telecom biz for 30 years, have thousands of DC battery plants with up to 14 parallel strings and no diodes in sight. Telco's have done it this way for over 100 years with no problems.

 

[Lxnxjxhx]

telecom

telecom

Lead acids, right?

 

[jdsmith]

I think this is being over thought. I have been in the telecom biz for 30 years, have thousands of DC battery plants with up to 14 parallel strings and no diodes in sight. Telco's have done it this way for over 100 years with no problems.

We did the same thing in a solar PV system. AGM batteries in our case, a special case of lead acid.

 

[mdshunk]

The only time I've used a diode in a battery bank was tapping the string partway for a lesser voltage. For instance, if you have a 36V bank, tapped at the 12V point for some 12V loads, and a cell or cell connector goes open, you could potentially send closer to 24V to your load at the opposite polarity. The diode keeps this from happening. Powering a 12V two-way radio on a 36V forklift or golf cart battery bank is one typical application for the diode in my example.

 

[Lxnxjxhx]

I haven't checked on this, but maybe lead acid batteries in parallel all charged beforehand to the same voltage can handle current sharing by themselves. I think nicads have very low internal impedance and may not handle this without circuitry. Maybe even using skinny wires works for current sharing.

I'll ask the AGM folks how they do it.

 

[dereckbc]

Lead acids, right?

Various forms of lead acid types from pure lead to AGM.

The point that is being missed here if you put a diode in each battery string, the circuit will only allow current to flow in one direction only. Which means the battery can only discharge, or charge depending on which way it is installed but not both. I must be missing something because I cannot see a way for it to work.

 

[Weaver Road]

I like where this went to!

I like where this went to!

To All;
I not only got my answer right away, but a lot of other information as the conversation developed. My biggest problem was being sure the sizing was right for the requirement of the system to carry over the 24 period of idle time and then 5 minutes of fire alarm activation. So now I know that two 12v 7Ah batteries in series still give 7Ah, at 24v.

Thanks for the speedy help guys (and gals?? maybe??)
:smile:

 

[Lxnxjxhx]

the nicad people haven't responded yet

the nicad people haven't responded yet

Can I parallel AGM batteries without some type of current
sharing circuitry?

From Concorde Battery Corp, the AGM people:
Yes you can, provided the voltage of each battery or string of batteries to
be paralleled is the same.

 

[dereckbc]

Can I parallel AGM batteries without some type of current
sharing circuitry?

From Concorde Battery Corp, the AGM people:
Yes you can, provided the voltage of each battery or string of batteries to
be paralleled is the same.

Sure you can. There is one catch. When you parallel batteries they must be compatible types with the exact same float, equalize, and capacity. For example you would not put a pure lead acid battery in parallel with a valve regulated type.

What is obvious is to parallel the same model battery with each other.

 

[LarryFine]

Some of my trucks have two batteries. They charge through a "battery isolator", which is basically two big-mamma diodes in one handy package.

Huh? If you put a diode in the circuit, depending on polarity, will mean the battery can only discharge or charge, but not both. What am I missing in your statement?

The point that is being missed here if you put a diode in each battery string, the circuit will only allow current to flow in one direction only. Which means the battery can only discharge, or charge depending on which way it is installed but not both. I must be missing something because I cannot see a way for it to work.

Dereck, what you're missing is that the dual-battery trucks do not parallel the positive terminals of the two batteries. One is for vehicle starting and the other is for auxillary loads. Sometimes, the two can be temporarily linked for starting.

The diodes are in line between the alternator output and the two positive terminals. Without diodes, you'd either be stuck with paralleled batteries or with the inability to charge both simultaneously. The diodes only carry charging current.

 

[Lxnxjxhx]

The Nicad people respond

The Nicad people respond

Yes, you can assemble Ni-Cad cells in parallel. Be careful not to mix batteries constructed from other than identical cells. Don?t mix cell types, cell sizes, cells from different cell manufactures, or, even manufacturing lots may give results ranging from simply unsatisfactory to disastrous.

The problem with parallel-connected batteries arises in charging. Parallel charging could easily results in improper sharing of charge current, resulting in battery damage. Sealed cell batteries should not be charged in parallel.

The best approach is to use a series/parallel switch or electronic system to allow only discharging in parallel.

 

[stickelec]

My experience has been that parallel-charging (and discharging) for Lead-Acid and Lead-Calcium is fine as long as all cables are consistant in awg and lengths.

I don't think that LaMarche, SENS, and C&D are aware of the problems you mention regarding the series/parallel charge/discharge routine you referenced.

Parallel operation is common when the desire is to provide more amp-hours.

 

[dereckbc]

The problem with parallel-connected batteries arises in charging. Parallel charging could easily results in improper sharing of charge current, resulting in battery damage. Sealed cell batteries should not be charged in parallel.

The best approach is to use a series/parallel switch or electronic system to allow only discharging in parallel.

Nonsense, there is not a single battery manufacture that would back that statement up. The fact is there are litterally 10's of thousands of battery plants located in any telephone offices, data centers, POCO generation/substations, etc'' that do exactly that for over a hundred years. Those industries use a wide range of battery technologies from pure lead, lead-calcium, lead atimony, AGM, to sealed valve regulated, etc...