NiMh Batteries in Series (12v) Charger Question

MikeML

Joined Oct 2, 2009
5,444
Back when I carried a Walkie-Talkie full time, the radios used NiCads which are similar to NiMh... I found that the battery life was maximized by having (at least) two battery packs. Put a fresh one on the radio, run it until it went dead while the other pack was on a 14hour (not fast) 0.1C charge. Swap batteries, and repeat.

I used a rotary 120Vac timer set to 1 hour out of 24h to turn off the charger. It was just a matter of rotating the dial on the timer so it would click off 14h after I dropped the dead battery into the charger. After that, the timer would charge the battery for 1h every 24h thereafter, until the other battery needed charging...

Lead-Acid batteries are much easier to deal with than NiCad or NiMh. Read the charging guidance on either BU or Powerstream
 

Thread Starter

Carlos Almeida

Joined Jan 15, 2015
35
If you go the timer controlled charge route - don't forget whatever percentage of charge might be left in the cells from last time - a full timed charge for cells that are only half discharged could result in cooked cells.

Some commercial "dumb" chargers that rely on a timer, have a discharge phase at the start - I don't know the exact procedure, but they probably just connect a resistor across the cells for another timed period.

The one I bought in the Lidl store before they started offering the MCU controlled ones, only does discharge monitoring on the cell nearest the control panel, so it could be a bit more sophisticated - maybe a comparator monitoring the terminal voltage that starts the timed charging period.
Hi,

Those chargers sound familiar to me, I'll do some digging to see if I can find one either in my garage or at an electronics store, seems like a very good solution to recharge these batteries in series.

Will update here what I can find, thanks.
 

Thread Starter

Carlos Almeida

Joined Jan 15, 2015
35
Back when I carried a Walkie-Talkie full time, the radios used NiCads which are similar to NiMh... I found that the battery life was maximized by having (at least) two battery packs. Put a fresh one on the radio, run it until it went dead while the other pack was on a 14hour (not fast) 0.1C charge. Swap batteries, and repeat.

I used a rotary 120Vac timer set to 1 hour out of 24h to turn off the charger. It was just a matter of rotating the dial on the timer so it would click off 14h after I dropped the dead battery into the charger. After that, the timer would charge the battery for 1h every 24h thereafter, until the other battery needed charging...

Lead-Acid batteries are much easier to deal with than NiCad or NiMh. Read the charging guidance on either BU or Powerstream
Hi Mike,

That rotary timer method is probably what I'll be using, looks exactly what I want given my (un)perfect project.
I'll also test the Lead Acid Battery to see how well it performs against the NiMh, reading all that information from Battery University and Powerstream it seems like a safer choice but it also seems that they discharge quite quickly when not being used ?

Thanks everyone for your support on this.
 

MikeML

Joined Oct 2, 2009
5,444
... reading all that information from Battery University and Powerstream it seems like (LA is) a safer choice but it also seems that they discharge quite quickly when not being used ?
SLAs have a fairly low-self discharge rate (lower than a comparable NiCd or NiMh), worst at elevated temperatures, not too bad at room temp. A re-charge every three months or so (if the battery is disconnected from any load) is sufficient.

I have an application (Arduino) where I draw ~50mA continuously off a 6Ah SLA. The float charger is carefully adjusted to hold the voltage at 13.60V, so delivers a couple of mA to overcome the self-discharge of the battery while holding up the 50mA load. In the event of an AC line failure, the load current comes out of the SLA, and then the nominal 1A charger recharges the battery when the AC is restored...
 

ian field

Joined Oct 27, 2012
6,536
Hi,

Those chargers sound familiar to me, I'll do some digging to see if I can find one either in my garage or at an electronics store, seems like a very good solution to recharge these batteries in series.

Will update here what I can find, thanks.
None of the units I have do serial charging - not even the "dumb" variety. They all have a row of individual battery bays with a common along one end.

If you want to charge a series string - you need them to be in a battery holder that you can take them out of and put in the charger bays.
 

Thread Starter

Carlos Almeida

Joined Jan 15, 2015
35
SLAs have a fairly low-self discharge rate (lower than a comparable NiCd or NiMh), worst at elevated temperatures, not too bad at room temp. A re-charge every three months or so (if the battery is disconnected from any load) is sufficient.

I have an application (Arduino) where I draw ~50mA continuously off a 6Ah SLA. The float charger is carefully adjusted to hold the voltage at 13.60V, so delivers a couple of mA to overcome the self-discharge of the battery while holding up the 50mA load. In the event of an AC line failure, the load current comes out of the SLA, and then the nominal 1A charger recharges the battery when the AC is restored...
Hi,

I was really convinced that SLA's discharged faster than a NiMh, great to know I was wrong, will go down this road then since will give me more amperage and this way I'll only need to recharge it once a month, will use some NiMh in series to build a smaller project (some nice little speakers for my mobile). This also takes some pressure of 10 batteries in Series and gives me more freedom to easily remove them from the Source and charge them normally through a 4 battery charger (which I already have). At least now I know the correct way of charging NiMh in series :)

Arduino...Arduino is the main reason why I recently got involved in circuits, the things we can do with it are endless and with the right effort into it we can actually build things that we only dreamed of. I'm starting with the basics first, batteries, diodes, capacitors, relays, etc just to be comfortable before playing around with an Arduino, but that's my main objective, enter 2016 with an Arduino project already built.
I know I could just follow instructions step-by-step but that's not really my goal, I want to learn and make my own version of things (or even new things), my way or the highway eheh

So, returning to SLA's, a proper charger is easily obtainable so that's solved, although I do have 2 questions that I couldn't find a proper answer to them.

1.) Can I physically place a SLA without the terminals facing upwards ? For example, laying the battery on it's side (permanently) would harm the battery? I know that for the old Lead Acid Batteries this would be totally out of the question and would probably be a very dangerous scenario, not sure about these new ones.

2.) Can I use the battery while it's charging? Basically my first approach (when I was going to use NiMh) was to place a Switch that would disconnect the main circuit/load from the batteries while the batteries were being charged, is there any way I can charge the SLA and have the circuit running at the same time?

Thanks once again everyone
 

Thread Starter

Carlos Almeida

Joined Jan 15, 2015
35
None of the units I have do serial charging - not even the "dumb" variety. They all have a row of individual battery bays with a common along one end.

If you want to charge a series string - you need them to be in a battery holder that you can take them out of and put in the charger bays.
Hi Ian,

That's the type of charger I have too, as I mentioned in my previous post, I'll skip the 10 batteries in Series and make a smaller project with 4 or 5 in series depending on the "consumption" needs of the circuit (+/- 5V) so it will be easy to just put them on the proper battery charger.

Thanks for the feedback
 

MikeML

Joined Oct 2, 2009
5,444
The only warning I have about SLAs is that it is bad for them to sit around un- or under-charged. They are not good for repeated deep discharges, nor for powering a low-power project continuously for long intervals between recharges. Used this way, on average, they will be less than 90% charged for long periods, which leads to sulphation that doesn't go away on the next recharge.

A better use model is the one I already described, where they are continuously float-charged with an AC powered charger, such that if the AC fails, they can deliver most of their capacity for a day or two, and then get recharged immediately when the AC is restored.

You always should top up any lead-acid chemistry battery immediately after use and prior to storing the battery. Then, while being stored, the battery should be topped-up at least once a month. Alternatively, keep a proper float-charger on it continuously while the battery is not being used...
 
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