connecting batteries in parallel or series, myth and theory

Interesting paper on the topic of parallel battery wiring here:

For me, as someone who lives off-grid, has a 48V battery bank, and does his own battery maintenance, the big advantage of having a single string, vs multiple parallel strings, would be the ease of maintenance -- 1 string =

24 cells to check; 2 strings = 48 cells to check, etc.

When I purchased my batteries, I could not purchase 2V batteries in the capacity I would have liked. So I have two parallel strings. But the 2V cells are now available in a satisfactory size, so when my present bank needs to be replaced, I will be using a single string.

Other advantages/disadvantages suggest themselves, but are application dependent.

--ron

I've done a fair amount of work on battery control circuits, which monitor voltage, current and charge state and shut off a bad cell if needed. A safe circuit isn't easy to design and test and an unsafe one is a serious fire/explosion hazard. WW2 submarines connected their batteries in parallel sometimes, with a full-time operator on watch. They kept a log of the specific gravity of each cell, though. I think it's possible but not worth the trouble.

I have been doing it for years with no ill effects, but I recognize that it is not an optimal configuration (currently two sets of two 6-volt batteries, formerly four 12-volt batteries). I protect against possible high circulating currents with fuses.

My batteries are there mostly for an emergency, so they seldom see a high DOD, and that may be why I have no problems.

Vaughn

Series parallel batteries.

First, whether you use 2V cells or batteries you end up with "A Battery"

The problem with parallel strings is that they do not all charge or discharge together.

When I was getting my accreditation for solar design and installation we were shown the results of a series parallel bank, if memory serves, of 36 x 2V cells. That is six parallel strings of six cells.

The up shot was that under charge or discharge the strings did not operate as a single battery. i.e. Under charge the strings would charge one at a time.

1 4 3 6 1 5 4 6 5

The same for discharge. There were also very high transient voltages across the array.

No, I no longer have the notes for the course, it was twenty years ago.

Parallel string should be avoided wherever possible.

If parallel strings can't be avoided you should try to use a cell that is large enough to not exceed two parallel strings.

Yes, there are situations that may make parallel strings unavoidable. One of the most common is space restrictions dictate the dimensions of cells to the point where you might be able to fit 12 / 500Ah 2V cells in a given space where you can't fit 6 / 1000Ah 2V cells. Another excuse that is used is that you can buy T105s cheaper than Surrettes.

Yes, A single string of series cell is the ideal. Parallel strings are the second best choice for battery storage systems.

ALL Batteriea are series connected cells. A "flashlight battery" is not a battery - it is a single cell. A 9 volt battery IS a battery, as is a 6 volt camera battery, or a lantern battery. Car batteries are too.

Definitely an advantage using one big cell over 2 smaller parallel connected cells. I'm no scientist so I cannot explain why - but they will last a LOT longer.

** Posted from **

Here's one of the simpler battery monitoring ICs that you could use to monitor the capacity of a series string of cells.

High-performance Lithium packs almost require electronic monitoring. The technology could be useful for a home battery bank of old, questionable lead-acid batteries so you'd know when one should be checked.

Accreditation? What organization has given you this accreditation? Or is this just something else you've made up?

--ron

| Interesting paper on the topic of parallel battery wiring here: | |

It seems from that, parallel string operation is mostly OK, with one notable exception being parallelizing to increase deliverable current capacity (as opposed to parallelizing to increase time capacity and reduce current per string). If one string fails under a high current load, it could be a big problem for the other string.

In alt.engineering.electrical snipped-for-privacy@gmail.com wrote: | On Aug 8, 4:43 am, snipped-for-privacy@ipal.net wrote: | |>

|> Who here thinks connecting batteries (one cell or multi-cell) in parallel is |> bad ... and why (if you don't know why, I'll assume it's myth). | | Series parallel batteries. | | First, whether you use 2V cells or batteries you end up with "A | Battery"

Right. But there is distinction of paralleling single cells individually versus paralleling a single battery of N cells, or a string of N cells. The latter do not have any cross connects at each cell connection. One consideration is whether there is any advantage to that. When I put some thought into it, I could not see any issue with lead-acid batteries.

| The problem with parallel strings is that they do not all charge or | discharge together.

If one discharges more, its voltage would drop, and others would take up the slack. At least that's the way I see it. A posted paper URL in this thread earlier suggested it's OK to parallel different strings or batteries with different capacities and they would discharge in proportion to their capacity. I'm guessing that is because of the voltage balance effect (the stronger string with the slightly higher voltage carriers more of the load).

| When I was getting my accreditation for solar design and installation | we were shown the results of a series parallel bank, if memory serves, | of 36 x 2V cells. That is six parallel strings of six cells. | | The up shot was that under charge or discharge the strings did not | operate as a single battery. i.e. Under charge the strings would | charge one at a time.

So from a deep discharge position (20-50%) a recharge would not bring each string back to 100% at the same rate, and if a discarge was needed before all of them are fully recharged, it would present an imbalance not in capacity, but in charge level.

Does that apply to both slow and fast recharge?

| The same for discharge. There were also very high transient voltages | across the array.

What would be causing a transient?

| Parallel string should be avoided wherever possible.

Why?

| Definitely an advantage using one big cell over 2 smaller parallel | connected cells. I'm no scientist so I cannot explain why - but they | will last a LOT longer.

Can you tell me where you get this information? Have you tried both? Or you just heard this somewhere?

I have never seen single cells paralleled. They are always connected in series to get the desired voltage. Then, another group of identical series cells may be paralleled with it.

mike

However, I believe they are mostly discussing systems that spend most of their time on float charge, with occasional

But the authors also described a situation where one string would discharge more than another. Those circumstances -- "... a system that is designed for long discharges, but is subjected to frequent shallow discharges" could describe many off-grid systems. The result is that one string, (in the paper the "high-rate" battery), receives the brunt of the cycling duty, and may age prematurely as a result.

This is probably not a significant issue with two matched strings that are placed in service together. But this phenomenon is, I believe, the source of the recommendation to not mix batteries of different ages in off-grid systems.

--ron

S.E.I.A.A.

Solar Energy Industries Association Australia

Now Known as B.C.S.E.

Business Council of Sustainable Energy

AKA - Boring Curmudgeons Spreading Entropy.

Ya hear the craziest things sometimes.

I've been running paralleled 12 volt batteries in my motorhome for a decade. Currently 5 group 29 batteries. As my battery -powered AC project progresses, there will probably be a couple more. The current set is about 2 years old and is charged by a single 80 Progressive Dynamics Intellipower/Charge wizard. Or when I'm doing extended dry camping, by my 150 amp cordless battery charger.

My house's UPS has a wide variety of sizes and brands, connected in series/parallel to produce 24 volts at close to 1000 amp-hours. Every time I find a decent-looking* deep cycle battery, I toss it on. The string is charged by a 20 amp 24 volt telco charger. I don't have any idea what the actual capacity of this conglomeration is but since it has lasted through the longest power outage I've experienced to date, I'm happy.

*that is, checks out with a 500 amp battery tester.

You can make this stuff as complicated as you want but as long as there isn't much money involved, why bother? If I were dealing with a few thousand dollars worth of Surettes or similar batteries, then yeah, I'd sweat the small stuff. But my RV batteries cost about $60 each when I got 'em and the ones on my UPS mostly came from the scrap metal yard at 20 cents a pound. I keep the RV ones watered and cleaned and the PD does a periodic equalization automatically. Other than water, I couldn't care less about the UPS batteries. When they quit, I'll get some more.

John

-- John De Armond See my website for my current email address

Except that I use 12 volts, this has been remarkably close to my own past strategy. A minor part of my job was to change out strings of batteries on our various UPS systems. The object of that game is to replace just BEFORE the first battery in any given string goes bad, so it was a minor deal to test a batch and then bring home the better used batteries.

Now that I am retired and may actually have to BUY batteries, my strategy may need to change.

Vaughn

Hmmm. Interesting. I note that you are NOT listed on the BCSE Accredited Designers and Installers List at

Searching for Solar Energy Industries Association Australia leads me to Sustainable Energy Industry Association(Australia) Ltd and the link to that web site redirects from to and insurance company website!

--ron

If you had asked I could have told you that. No secret there. I haven't been a member for perhaps seven years, maybe eight.

from

and insurance company website!

I told you they became BCSE.

What you won't find on their site is the fact that the BCSE is teetering on the brink of revolt from the members. Has been for several years.

The entire industry is dead in the water. Full of cowboys and government spin doctors. Sad really, OZ had the chance to lead the world in solar power use and threw it all away. Now it's all yuppie driven status seeking.

Still, the cowboys keep me working, I haven't advertised for years and I still get phone calls from people who can't get their installer to return phone calls or show up to repair their stuff ups.

Starting batteries are often paralleled (though have isolation diodes) for higher current. Most large trucks have such a setup.

I have often seen truck batteries connected in parallel, particularly on diesel pickups, but have never noted isolation diodes.

I have also seen one at least one near-truck fire that was apparently caused by shorted cells in one of the paralleled batteries on a truck, which led to the good battery forcing a high charge current through the bad battery. Ironicly enough, it was a fire truck.

Vaughn