I have 2x12V 270mAH NiMH batteries wired in parallel to vive me a nominal 12Vx5400mAH battery for one of my locos.

On another forum I was given advice that I had to put diodes between the batteries to prevent the possibility of one going faulty and the other trying to charge the faulty battery.

I also received contradictory advice so I am asking if anyone has a similar set up and do they use diodes?

If I used diodes I was advised that the batteries had to be charged separately and I have seen differing opinions about this as well.

Anyone have experiences with charging batteries connected still connected in parallel and charged together?

Even a google search turns up conflicting opinions so I guess I am after someone who has done this.

What chemistry? I’m not sure that matters, but it might.

I have one loco that has two 2 cell (2S) LiIon packs wired in series to increase voltage. I charge them together at the total voltage (as one 4S pack). I realize this is not the same as two parallel wired packs. I have no direct experience with that.

It makes sense that if you install back feed prevention diodes you need to charge individually. You also need to attach the charger at the pack before the diodes or they will not charge.

Putting the packs in parallel does not guarantee that they get equal charging current.

If they are different enough, you can overcharge one of them.

Best to wire them up so they can be charged separately.

Greg

I agree with Greg.

Charge separately. Use diodes to prevent one pack back feeding into the other.

Stop! If your not sure on giving solid or accurate advice…please don’t.

Mr. Price, you can wire two Nimh battery-packs in series and/or in parallel. But you need to make sure of a couple of things before you proceed.

Wiring two identical battery-packs in series will double the voltage but will still have the same cell capacity. Wiring two identical battery-packs in parallel will double the capacity, but will still have the same voltage.

You can connect identical voltage Nimh packs of the same capacity in parallel, but only if they are the exact same voltage, chemistry and cell manufacturer and cell type. You also need to make sure that only one pack has a temperature thermostat welded inside. Having two battery-packs of the same voltage and chemistry with two separate thermostats will charge and discharge one pack more than the other.

Before you connect battery packs in parallel, you must make sure that they are both charged to the same voltage.

You do not need a diode!

This procedure above is only for Nimh or NiCd battery-packs.

Rick Isard said:

Stop! If your not sure on giving solid or accurate advice…please don’t.

Mr. Price, you can wire two Nimh battery-packs in series and/or in parallel. But you need to make sure of a couple of things before you proceed.

Wiring two identical battery-packs in series will double the voltage but will still have the same cell capacity. Wiring two identical battery-packs in parallel will double the capacity, but will still have the same voltage.

You can connect identical voltage Nimh packs of the same capacity in parallel, but only if they are the exact same voltage, chemistry and cell manufacturer and cell type. You also need to make sure that only one pack has a temperature thermostat welded inside. Having two battery-packs of the same voltage and chemistry with two separate thermostats will charge and discharge one pack more than the other.

Before you connect battery packs in parallel, you must make sure that they are both charged to the same voltage.

You do not need a diode!

This procedure above is only for Nimh or NiCd battery-packs.

The two “batteries” were manufactured by myself from tagged AA cells bought at the same time from the same supplier/manufacturer.

After assembly the2 batteries were charged separately using my smart charger and then connected in parallel to make a bigger “battery”. (I am using this term to describe the 2 batteries wired in parallel)

At this point in time the bigger “battery” still measures a nominal 12V on my DMM but there will come a time when I will have to charge it.

From my understanding a AA cell when connected to another cell makes the result a battery, so 10 connected in series make a 12V battery and 2 of these connected in parallel make a battery with increased capacity.

To the best of my knowledge there is no temperature thermostat.

X

Graeme.

For the sake of a couple of cheap diodes you will remove any doubt as to safety.

Here is how to wire them and charge them separately in situ.

Incidentally, if the packs have been put into parallel to gain extra capacity for longer run times please remember that the same principle of current drain from AA cells will still apply. Any greater draw than 1 amp with AA size packs will result in a much reduced life span of all of them.

Mr. Price,

That is correct. If you not sure, just give me a call.

I agree stop! But before bandying words about like “expert”, you might want to explain some of your statements.

Why, pray tell, do you say that 2 packs in parallel and one with a thermostat works, but 2 packs each with a thermostat don’t.

I have several engineering degrees, so please give an explanation rooted in laws of physics and electrical engineering. I also have 2 people with PHD’s in the subject working for me in case I cannot understand the explanation.

Regards, Greg

Rick Isard said:

Stop! If your not sure on giving solid or accurate advice…please don’t.

Mr. Price, you can wire two Nimh battery-packs in series and/or in parallel. But you need to make sure of a couple of things before you proceed.

Wiring two identical battery-packs in series will double the voltage but will still have the same cell capacity. Wiring two identical battery-packs in parallel will double the capacity, but will still have the same voltage.

You can connect identical voltage Nimh packs of the same capacity in parallel, but only if they are the exact same voltage, chemistry and cell manufacturer and cell type. You also need to make sure that only one pack has a temperature thermostat welded inside. Having two battery-packs of the same voltage and chemistry with two separate thermostats will charge and discharge one pack more than the other.

Before you connect battery packs in parallel, you must make sure that they are both charged to the same voltage.

You do not need a diode!

This procedure above is only for Nimh or NiCd battery-packs.

Greg,

Manufacturing two identical Nimh packs with Pepi 70 - 75 degree thermostats is not a problem. But when you wire those two same packs in series or in parallel with one another and both packs have a thermostat installed within, one pack will be fully charged before the second pack reaches it’s full capacity.

We have tested this several times, with three different cells and manufacturers and all come to the same result. The last set of 3 - 5 cells in a 7.2V Nimh AA pack do not reach their fully capacity or voltage. This was due to the first thermostat reaching it’s thermo cut-off, stopping the current to the second pack. I do have several retired Rockwell / Collins engineers on my staff, whom all are quite a bit smarter then I and all come to same conclusion.

Because, one has a PHD does make that person smarter then another, it just means they paid more for their education then I.

First off, the people I have encountered in engineering KNOW MORE when they have spend MORE time LEARNING.

Achieving a PHD means you spent more time learning and you had to do original research and pass a review by people who know more than they do. It does not make a person smarter or wiser, but neither of those are synonyms for KNOWLEDGE.Your response that a PHD is merely more money spent is silly.

Even though I asked carefully, you don’t give any factual reasons or information in your first paragraph.

In your second paragraph, you actually have several fatal blunders.

First we are talking ONLY of packs in parallel, yet you bring series into the conversation. Besides confusing the issue, you make additional blunders.

Yes of course if you have all the cells and thermostats in series, one thermostat opening shuts it all down. DOH, that is what series is all about, any break in the chain stops all current. Agreed.

In parallel, what this thread is about, if one thermostat opened then the other pack would continue charging, at most likely too high a rate if you don’t have a smart charger that limits current intelligently.

But you don’t even describe EITHER situation correctly, you state:

But when you wire those two same packs in series or in parallel with one another and both packs have a thermostat installed within, one pack will be fully charged before the second pack reaches it’s full capacity.

COMPLETELY WRONG! FOR SERIES, ALL CURRENT STOPS, this is the definition of a circuit in series, all current goes through a single path in all components.

And in the case of parallel packs charging and both have thermostats and one opens, YES, THANK YOU FOR REINFORCING THAT TRYING TO CHARGE 2 PACKS IN PARALLEL IS NOT A GOOD IDEA.

I’m glad you admitted that this is not a good idea. I won’t go further about how only one pack having a thermostat works, you have enough errors here to chew on. I suggest you read my response to your retired engineers, and if they think I’m wrong, then it’s a damn good thing they are retired.

Greg

Rick Isard said:

Greg,

Manufacturing two identical Nimh packs with Pepi 70 - 75 degree thermostats is not a problem. But when you wire those two same packs in series or in parallel with one another and both packs have a thermostat installed within, one pack will be fully charged before the second pack reaches it’s full capacity.

We have tested this several times, with three different cells and manufacturers and all come to the same result. The last set of 3 - 5 cells in a 7.2V Nimh AA pack do not reach their fully capacity or voltage. This was due to the first thermostat reaching it’s thermo cut-off, stopping the current to the second pack. I do have several retired Rockwell / Collins engineers on my staff, whom all are quite a bit smarter then I and all come to same conclusion.

Because, one has a PHD does make that person smarter then another, it just means they paid more for their education then I.

Oh, carp.

First off, I’ve been doing this (commercially) for a lot of decades. Early on I obtained a Sanyo Engineering Manual (stupid engineers, anyway), and they warn to not place the batteries in parallel. With caveats.

I won’t do it. The chances of one of them there silicone protection devices failing is there.

If I need two sets (and in the early days I did), I had a switch, sideways throw, A and B packs. Charge one, flip the switch, charge the other.

When running, one pack dies, look at yer watch, geez, now you know how long it ran…second set if identical should do the same, flip the switch and off you go.

Hook them up in parallel. I will make a note.

Don’t ever come to any forum, as in ever, and complain that you A) burned your house down, B) burned your garage down, or C) melted your engines.

Now, we did series-parallel on lead acid wet cells in one job I had…from the maneuvering room, with the sticks…but that was qualified professionals (really qualified) and constant monitoring. Still, poop happened…ask the guys on the Cochino.

Back to my series-only battery packs.

TTFN

TOC

The fundamental issue is that the internal resistance of a battery pack is low. So minor variations can make enough difference to charge one pack way more than another.

Also, as Rick pointed out, one pack developing an open can put too much current in the other pack, although I would probably size the charger for a single pack so that 2 packs charge half as fast if every thing is ok, and full current to one if the other fails.

Of course, I have never liked the “half-fast” (say that quickly) solution, and that’s why I just say do not parallel two packs to charge with a single charger.

Greg

(half-fast, get it?) (http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-cool.gif)

So the best solution, if possible/practical, would be to get one pack with twice the capacity of the packs in question. Then there is no parallel to worry about.

Am I right?

Depends. Sometimes space is a consideration, sometimes folks want to know when they’re halfway through…and have a reserve, like on you old Volkswagen Bug.

In the old days, we were limited in MAH capability, and even now, I can (and sometimes do) put a 4500MA pack in, a switch, and another 4500MA pack.

My rule of thumb has always been…how long will your knees last?

Batteries should be just a tad longer and you’re good to go.

Around here, when I never know what usage a particular loco will get in an ops session, I overkill a bit.

And they work…including the 700MA nicads in an OLD Bach Porter that are so bad we have to charge them twice before an ops session starts, and they run that engine all night…including use by visiting kids who ran it and ran it and ran it.

But this whole thread is about paralleling battery packs, and I just won’t. Switch is better. You KNOW each pack has charged fully off your smart charger.

Since they sense volts and current, no idea how that would react when one is done first and it sense that.

If I recall from my reading many moons ago, Delta…a cross of voltage and current…not a specific number, but where it crosses, so one could be up through diodes, one not, and you get oh, 3 hours on one pack, and one on the other.

My thesis is: Don’t try to out-think the technology, and re-design the function on a SHK University degree.

TOC

Hi Dave, Yes I would recommend one pack at the rated amperage you desire. I basically translate amperage into runtime. 1 amp to 1 hour , 2 amp to 2 hour, etc. Of course it helps to know what loco you are using and I can guestimate your runtime.

Here is a good one. I have a customer in central Florida who has a 16.8v 4000mAh nickel metal hydride battery pack in a box car for long runtime times. He did a test running his LGB Mogul and 7 cars. The train ran for 7 hours! Based on the specs it should have stopped around 4 hours.

So it is possible to get really long runtimes with a standard battery. This was a few years ago. Since then Lithium packs have increased in amperage and the cost is below most nickel metal hydride packs. So the selection of batteries has expanded.

I do not recommend do it yourself battery packs. I am in the business of making you happy running your trains.

Don

RCS of New ENGLAND

Multiple like type and condition batteries utilized in parallel aren’t inherently an issue herein, charging them stacked up in parallel is the concern. Individually charging each battery as suggested by others is the answer to this conundrum.

Best case for battery charging, performance and long life of NiCD/NiMH technologies is a soft peak charge, completing the charge process with a low/slow trickle charge for 12-16 hours (allows the cells to equalize/balance absorbing potential without creating undesirable heat). The need for a so called smart chargers for NiCd/NiMh technology is NON existent… A Trickle and or Peak charger is all that’s really required. Cycling these batteries on occasion is well proven to benefit the life expectancy and performance of these cells.

The reason a 4000mAh battery can run a train for seven hours is easily factored with math and the average current draw of said engine under load. In theory a 4000mAh or 4 Ah battery should provide 4 amps at rated voltage for one hour. If one plotted a discharge curve you’d note the battery MAY provide 1 amp for four hours @ 16.8V, but continues to provide power well beyond said number. 16.8V NiMh battery will provide voltage and current all the way down to 12.0 volts if not lower… In the described circumstance 4 hours at 16.8V and then three hours more with voltage degradation in play.

Michael

Michael Glavin said:

Multiple like type and condition batteries utilized in parallel aren’t inherently an issue herein, charging them stacked up in parallel is the concern. Individually charging each battery as suggested by others is the answer to this conundrum.

Best case for battery charging, performance and long life of NiCD/NiMH technologies is a soft peak charge, completing the charge process with a low/slow trickle charge for 12-16 hours (allows the cells to equalize/balance absorbing potential without creating undesirable heat). The need for a so called smart chargers for NiCd/NiMh technology is NON existent… A Trickle and or Peak charger is all that’s really required. Cycling these batteries on occasion is well proven to benefit the life expectancy and performance of these cells.

The reason a 4000mAh battery can run a train for seven hours is easily factored with math and the average current draw of said engine under load. In theory a 4000mAh or 4 Ah battery should provide 4 amps at rated voltage for one hour. If one plotted a discharge curve you’d note the battery MAY provide 1 amp for four hours @ 16.8V, but continues to provide power well beyond said number. 16.8V NiMh battery will provide voltage and current all the way down to 12.0 volts if not lower… In the described circumstance 4 hours at 16.8V and then three hours more with voltage degradation in play.

Michael

Oh, no.

We went through this on another LS forum years ago.

You can slow charge NiMH, but you need to calculate the rate, know the MAH capability of the cells, and time it…and then, you’d best know for darn sure where the charge state is.

I didn’t read where anybody said you needed a smart charger on NiCads, but…

There was this guy, Cincinnati GR convention, who had NiCads (very forgiving technology), and powered track for charging with a very high rate NiCad charger.

Somebody talked him into putting NiMH in for the convention.

50 people in the patio, on the tour, and they hear a “WHUMP” swish-swish-swish as the loco on the parking track, charging, blew up, and the long hood of the diesel came flying through the trees and landed in the middle of the attendees.

Gee, he couldn’t figure out why.

Well, if I had 12-16 hours to charge 14 locomotives before an ops session, maybe I could do that…IF I had run them all flat.

I had a stack of about 200ma slow chargers…took them all out.

I have three Smart Chargers, can do NiCads and NiMH, do 14 locos and a dozen sets of back up batteries in case someone forgot to charge their engines that they bring (happens), all the day before a run (except the one engine).

We do fast charging with smart chargers because we have a lot of engines, don’t want to invest in 14-20 slow chargers, don’t have the time to wait for them to come up, and we never know the state of the charge.

You DO need a smart charger with NiMH, and I wish you well with your slow rate chargers.

Oh, and the old LGB catalog stated that Moguls drew .6A. Run time is about right.

TOC

Dave,

Obviously the step differential charge approach I alluded to requires knowledge of battery type, cell count, capacity, charge rates and time allowance. Same as any other charge algorithm in play. Note, I did not suggest slow charging NiMH batteries…

Who knows why the guy at Cincinnati GR convention vented his cells, probably because he used the wrong charge algorithm, charge voltage/current and or rate. More likely than not the issue was operator error, which is incredulously common…

Both NiCd/NiMH work with the same basic peak charge algorithm or “NDV”, the ability to sense, see or measure the V bump at the peak spec is more sensitive (5mV NiMH vs 10mV NiCd per cell is the standardized specification as I recall).

Note: A dedicated NiCD peak charger is NOT compatible with NiMH cells, while the inverse is just the opposite.

I suggested best case charge scenario was soft peak with trickle charged topping. No you don’t have to go there every time you want to play with your toy trains, I should have noted same. And no, you don’t need multiple slow chargers either, even my old peak chargers can serve multiple roles, i.e., trickle and peak charging of NiCD/NiMH and more…

What does a smart charger perform or do that a plain old peak charger doesn’t do for NiMH? Perhaps we need to define SMART charger…

I’m not new to these batteries and chargers, I have lots of experience herein, and was involved in the R/C industry evaluating, using and abusing same for many years. You’ll find my recommendations are quite sound if you do some research.

Michael

Mr. Prices question has been asked and answered a long time ago, let’s move on!