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Marez on Parallel battery pack operation

To all you people that are operating batteries in parallel.

High Flight Magazine

International Miniature Aircraft Association

Winter Issue 2005 Vol 26 No 4

Electronics Ala Mode by Eloy Marez

e-mail: snipped-for-privacy@aOl.com

PARALLEL BATTERIES, LAST TIME AROUND!

Some of you will remember I previously wrote here on the subject of the common practice of using two batteries to power the receiver and servos. I shared with you what you can expect to happen if one of the batteries developed a shorted cell, in that the good battery will attempt to recharge it.

My tests, using a calibrated commercial grade (Fluke) meter, gave a current reading of three amperes.

That is all - I did not have any further comments on the subject, pro or can. However, the matter was opened on one of the Internet forums. It was obvious that some had not read my original comments; others simply did not understand what they read.

My test can be duplicated by anyone with an ammeter and some batteries, one a fully charged 4 cell unit and one three cell, charged or otherwise. You may not read exactly three amps, since the series meter in this case is effectively a resistor, its value and effect differing from type to type. But in all cases, you will see a high current that will rapidly deplete the good battery.

I chose not to get engaged in the Internet forum controversy, but I did some further research on the subject, and while I did not, nor expected to find anything on this particular R/C application, I do have some expert opinions on the use of parallel batteries to share with you. Expert that is, if you will accept the word of the engineers, who work at the companies, who provide us with the high quality batteries already mentioned, and who produce their engineering, application and technical manuals.

First up to bat, Sanyo Electric, who should need no introduction and who we should believe if we believe anybody. In Sanyo's Engineering Manual, on parallel

batteries, you will find: "When a battery which has abnormally low voltage is used, caused by a short or other deviation, the high current which flows into the battery may generate heat, bum the lead wire, and eventually damage the device in which it is being used" This brings up further food for thought... which I had not considered, high current generates heat. That is how your wife brewed your coffee and made your II toast this morning. It is rare, but airplanes have been known to catch on fire in the air. Though we have always attributed the incident to being exhaust and fuel related, another possibility has just emerged. No, I didn't survey that either.

While this all started with NiCd batteries, I am sure NiMH's are also being used in this manner. On that subject, Sanyo, who uses the trade name "Twicell" for its NiMH's has this to say: "Do not connect two Twicell batteries in parallel as this may cause leakage - of battery fluid, heat generation, bursting or fire".

Some of the companies I will mention may not be known to you, but remember that a huge electronics world exists out there outside of R/C. Saft America, in its literature states: "If batteries (NiCd's - em) discharged in parallel, use a protective device to avoid back dischcharge from one battery to another". Power-Sonic Corporation comments: "If batteries (NiCd's) are discharged in parallel, use diodes to prevent back discharge from one battery to another".

GP Batteries, a major battery supplier, writes: "We do not recommend parallel charging or discharging. Parallel charging will produce irregular charging current and back discharge from one cell another". One of my (many) electronics textbooks, "Basic Electronics", by RCA's Bernard Grob, (Library of Congress No.76-141919) touches on the subject, though he refers to cells instead of batteries. You are reminded that cells are individual units, batteries are two or more cells connected in series. Mr. Grob writes: "Bad cells should not be connected in parallel with good cells, however, since cells in good condition will supply more current which may overload the bad cells. In addition, a cell with a lower voltage will act as a load resistance, draining excessive current from the cells that have higher output voltage".

There are others, but how many authoritative sources do we need? They all back up my original statement, that if one cell goes bad, further more serious problems will result. And I still maintain that those that are using them without serious consequence are doing so because of having chosen high quality batteries, and have been lucky. And I further maintain that a better method is one of the true battery backers available from Cermark, Electro-Dynamics, or Electronic Model Systems. The subject is closed, I promise!

  • * * * * * * *

And you wondered why the IMAA is losing membership?

Reply to
Red Scholefield
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Red, Just a thought - what about when the packs themselves contain paralleled cells? Isn't some inherent risk involved there? Bill(oc)

Reply to
Bill Sheppard

Its the same thing, Eloy doesn't seem to grasp that. He is hung up on the subject not realizing that we are only saying discharge in parallel, not charge. People have been running packs in parallel for years, both for receiver/servo power and in electric flight for longer duration and less individual pack loads.

Red S.

Reply to
Red Scholefield

| Its the same thing

Well, the only places where I see batteries put in parallel that can't be removed from parallel for charging by people who know what they're doing are with LiPos and Pb batteries -- not NiCd and NiMH cells. So it's not _quite_ the same thing.

(Putting Pb/LiPo cells in parallel is perfectly fine, for charging and discharging, as long as they're at the same level of charge when you connect them.)

| Eloy doesn't seem to grasp that. He is hung up on the | subject not realizing that we are only saying discharge in parallel, not | charge.

And to be fair, all the warnings he gave were about discharging (and some included charging as well. Charging NiCd/NiMH cells in parallel causes other problems.)

| People have been running packs in parallel for years, both for | receiver/servo power and in electric flight for longer duration and | less individual pack loads.

Well, there is some truth to what he said. In fact, I don't recall seeing anything that was really blatantly _wrong_ with his post, beyond his claim that my wife made me coffee this morning!

However, his claim is a bit simplistic, as he forgot a few things --

1- in my experience, the most common failure mode of a NiCd/NiMH cell is to develop high internal resistance (perhaps infinitely high) rather than to develop a short circuit. In this case, parallel packs are just perfect, and will save your plane.

I can only think of one case where a NiCd or NiMH cell of mine spontaneously developed a short circuit. I can think of a few cases, however, where they developed a short circuit after physical damage (not unexpected!), and I can also think of a few cases where they've developed open circuits or lost most of their capacity in a very short time (like during a day of flying, or even a single flight.) Playing the odds, two packs in parallel seems a lot safer than one, even if you assume that a shorted cell with parallel packs = disaster (and it's not, read on ...)

2- if you do short out a cell (well, replace it with a shunt) in two parallel four cell packs, you may very well get 3A (depends on the cells and how long/thick the wires between the two are and the connectors) of current flowing, and yes, that will cause heat. However, in many cases this high current will cause the short circuit to open up again -- the problem will correct itself, at least to some degree. Granted, you'll want to get this pack out of service ASAP, but you'll probably finish out your flight without even noticing the problem.

And really, cells don't usually just short themselves out spontaneously (physical damage is another story.) Instead, they start self discharging at a very high rate -- not a complete short circuit, but perhaps close. So instead of 3A, we get 1A or so once that cell completely self discharges. Better.

3- even if the short circuit doesn't correct itself, the 3A flow won't last long, as the good pack will be discharging and the rest of the bad back will be charging. Eventually the voltages of the two will meet.

4- yes, planes can catch fire in mid-air. However, if your single pack, currently at 5.0 volts, suddenly loses a cell, you're now at 3.7 volts or so, which probably means that you no longer have any control whatsoever, and your plane is going to crash.

Which is better? Your parallel pack setup with one shorted cell, pushing 3A through the two packs and delivering about 4.5 volts to your RX and servos -- low, but enough to have control, or your single pack setup with a shorted cell, delivering 3.7 volts to your RX and servos, not enough to keep control.

The first scenario might start a fire, might melt some connectors, or the voltages might stabilize before this happens, but you'll probably still have control, as long as melted wires don't cause a short circuit. The second scenario generally turns your (giant scale IMAA!) plane into a lawn dart. Which would you prefer -- 80% chance of landing OK (parallel packs), or 20% chance of landing OK (single pack, short near the beginning of your flight and noticed right away) ?

If I had a giant scale IMAA plane, I'd probably go for two 5 cell packs in parallel, with diodes to prevent cross charging. The extra cells would overcome the voltage drop caused by the diodes, and I'd need to make sure I had diodes that were large enough for one pack to handle the power needs for the entire plane under peak usage without overheating. But I usually fly smaller planes, so I just go with two smaller packs in parallel, or just one pack.

| > Just a thought - what about when the packs themselves contain | > paralleled cells? Isn't some inherent risk involved there?

Reply to
Doug McLaren

Hi Doug, No wonder your wife doesn't do coffee and toast - you *don't* seem to understand what don't means! ;-))

Reply to
Ed Forsythe

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