NiMH or Lithium?


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Grogster

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Posted: 06:55am 21 Mar 2023      

Prices have got to a stage that one 9v NiMH rechargeable battery is within a few bucks of a 9v Lithium battery.

Is it now a case of six of one and half a dozen of the other, to support rechargeable batteries in certain situations?

I'm talking about backup batteries for the call-points I produce.

Usually, I use a NiMH 9v battery, which is trickle-charged over time to keep it ready for use.
Trickle-charging for years on end will eventually kill the battery, so should I just now be using 9v Lithium batteries do you think?
Lithium batteries can't be recharged, but they have an exceptional shelf life, so they will last for about the same time as a NiMH battery being trickle-charged for years if you see what I am getting at.

What do the members here think?
Smoke makes things work. When the smoke gets out, it stops!

OA47

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Posted: 07:38am 21 Mar 2023      

Grogster, what about the extra charge circuitry needed?

0A47

Grogster

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Posted: 07:59am 21 Mar 2023      

For NiMH, sod-all.
One charge resistor, and a diode-gate.

Mixtel90

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Posted: 08:22am 21 Mar 2023      

Sealed lead acid. Always works. Almost bomb proof. :)

They all have problems, either operationally or environmentally. There is no ideal solution.

I would say that, if the mains supply is pretty reliable and only short disruptions are likely then non-rechargeable lithium is the way to go - but recycle them. You should get somewhere around 8-10 years life before they need to be changed.

If the mains supply is less reliable then you need a rechargeable solution. NiMH is still a good system for this.

SLA is still necessary for maximum reliability under hostile conditions. It won't give the high current surges that lithium will, but it's safer and easy to float charge. Many thousands are still in use. Environmentally they don't score well though.

OA47

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Posted: 08:29am 21 Mar 2023      

  Quote  Grogster, what about the extra charge circuitry needed?


I meant to accommodate the LITHIUM

0A47

Grogster

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Posted: 08:48am 21 Mar 2023      

None.
No charge ability if I go Lithium.
Just backup supply.

Quazee137

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Posted: 09:06am 21 Mar 2023      

What is the drain like? I have been using some 1.5V that have usb charge built in.
  There are 9V that do the same. Might be a way to go even if having to recharge
  every few months or so.

 Something like this 9V recharge able


 Im looking at 5.5V 4F caps as a temp brown out fix.

Quazee137

Mixtel90

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Posted: 09:26am 21 Mar 2023      

A lot depends on installation conditions with batteries, particularly the temperature. Ordinary alkaline cells can make excellent backup supplies here in the UK as they are relatively cheap and will operate down to low temperatures, where some other chemistries start to lose their capacity.

Andy-g0poy
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Posted: 02:30pm 21 Mar 2023      

The key will be the usage of the battery.

Primary cell Lithium, which is what I think you are talking about is designed for very long operational life at very low current. In such cases I would be looking at the larger cell formats if you can fit them. If the current requirement over time is very low then 20 year lifetimes are possible.

The problem comes if the system is a bit power hungry. If the mains fails and the battery is use then, the lifetime of the primary cell may be compromised. At that point you need to consider rechargeable systems.

As has been suggested SLA is used in industry, and works well. You could consider rechargable lithium, battery charging is not a big issue there are dedicated chipsets to do that , but keeping it charged will require some form of active management, as trickle charging is a non starter. So a daily/weekly/monthly charge system, or recharge after use would be needed.

Define your current and power usages first, then see what fits best.

Andy

Grogster

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Posted: 02:06am 22 Mar 2023      

Thank you for all the replies.

...and welcome aboard, Andy.

The units only draw power when they transmit, and that is only sporadically.  Many of them can go MONTHS without a single transmit cycle, as they are only used when needed.

This is what has got me thinking about using Lithium batteries as a backup rather then the NiMH I use now.  The long-life Lithium batteries are now CHEAPER by almost six bucks per battery, then the rechargeable NiMH type.

Mains power is very reliable.  Sometimes there is an outage, but it only lasts for a few hours at most - they're pretty good at getting the juice back on again if it fails.  Not counting planned outages, the power can be reliably on all year round, so the battery only needs to catch the occasional power failure.

The units monitor the battery voltage, and can send a low-battery alert now, so I would naturally keep that in place, but remove the trickle charging resistor from the design.  Or maybe leave it there, but connect via a couple of solder-blob pads, then you can use either type.

Wouldn't be talking about this a few years ago, cos Lithium batteries were still so bloody expensive(for quality ones), but they have come down a lot in price over the years, and are at a point now, where they are CHEAPER then the rechargeable type, but they have the advantage of possibly outlasting a NiMH on trickle-charge over a long period of time.

Yes, when they are flat, you have to replace them, but after ten years or so of constant trickle, most NiMH batteries also need replacing cos they won't hold a charge anymore, so this is what set me off thinking about switching.  That, and I also have just recently changed the designs to use RP2040 modules due to lack of PIC32 chips, so I'm already making changes - why not the battery concept also? (rhetorical)

  Mixtel90 said  I would say that, if the mains supply is pretty reliable and only short disruptions are likely then non-rechargeable lithium is the way to go - but recycle them. You should get somewhere around 8-10 years life before they need to be changed.


That's kinda what I was also thinking and I think is how I will now proceed, but I wanted to post a thread, to see if it was just me thinking that.  

Thanks to all.

robert.rozee
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Posted: 04:08am 22 Mar 2023      

  Grogster said  The units only draw power when they transmit, and that is only sporadically.  Many of them can go MONTHS without a single transmit cycle, as they are only used when needed.


does the backup battery not also power the MX170 or pico during a power outage?


cheers,
rob   :-)
Edited 2023-03-22 14:09 by robert.rozee

Grogster

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Posted: 04:52am 22 Mar 2023      

Yes, it does.  Perhaps I should have worded that better: The units only draw any real power when they transmit.  While in standby, they only draw 5mA or so.  Many of them can go MONTHS without a single transmit cycle, as they are only used when needed.

robert.rozee
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Posted: 05:18am 22 Mar 2023      

if a power outage is only half an hour long, and during it there is a 30 second long transmission, then the current draw (at 100mA) averages out to the equivalent of less than 2mA. this pales by comparison to the draw of your MX170 (5mA), or pico (25mA at prompt, connected via USB).

with a pico, i could see any extended outage putting a dent in 9v lithium battery (1200mAh). for instance over a weekend (midday Friday to midday Monday) 20mA x 72 = 1440mAh, which would see the battery drained. even just a 1 day outage would drain 1/3 of the battery's capacity.

i'd be looking at sticking with the NiMh battery, but having the MX170/pico take note of when it was changed and sound a 10-year alarm to remind that a new rechargeable battery should be fitted. i assume your units can get the time from somewhere (perhaps transmitted over the base HC-12)?


cheers,
rob   :-)


addendum: you could look at using a LiFePO4 cell and a small boost converter, these can have a quite long service life if treated well, but the extra complexity would largely nullify any cost savings. even regular 18650 Li-Ion cells can have a remarkably long service life these days.
Edited 2023-03-22 20:41 by robert.rozee

Godoh
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Posted: 06:12am 22 Mar 2023      

One thing that I have found with Lithium batteries is that they don't like being cold. It may not be cold where you are but on higher current usage ( my brushcutter) the batteries take quite a while to warm up when the ambient temperature is under 5 degrees.
I use the brushcutter and it runs for a short time then stops, it does this for about 5 minutes or more depending on how cold the air is. Then once the batteries are warm it keeps running well.
That may not be a problem where you live, or for so low a current draw, but in Tassie Lithium batteries take a while to wake up.
Cheers
Pete

Grogster

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Posted: 07:58am 22 Mar 2023      

Good points, Rob.
Perhaps rechargeable 9v was the best decision after all....

I did look at 3v7 LiPo batteries(the ones in the silver bag) with a boost controller etc, but I agree with you - it adds an extra level of complexity, and one thing I did build that used them(foal alerts for horsies), they would fail for no apparent reason, and then the unit would not work, so I went back to 9v NiMH for that project too - never had a problem since.

@ Godoh - yes, it does get chilly here in winter.  Zero or less most nights.  Might stick with the NiMH batteries then.

robert.rozee
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Posted: 11:26am 22 Mar 2023      

oops... i wrote "midday friday to midday sunday", when i should have put "midday Friday to midday Monday". and "LiFePo" instead of "LiFePO4". thankfully it seems that with a little bit of jiggling one can edit past posts  

fyi: LiFePO4 cells (Fe for ferrous) have a 3.7v maximum charge voltage, and a nominal 3.2v terminal voltage. LiFePO4 is what the current MG ZS EV car sold here in NZ uses, in the form of a 51kWh pack. electrically far more robust, and greater charge cycles, but slightly poorer energy storage per kg. the long-range model, i have heard, uses a 72kWh conventional Li-Ion pack. you can also buy 2-packs of 14430/500mAh (link) LiFePO4 cells from Bunnings for $7.40. or 14500/600mAh (link) 2-packs for $12.

i'd shy away from any sort of pouch ('silver bag') cells, far more prone to being damaged and as a result having a 'poor customer outcome'. fully metal encased is much safer in my opinion.


one enhancement you could look at is using a pin on the MX170/pico to control charging. so, for instance, every week charge the NiMh battery for a minimum of 1 hour or until the terminal voltage reaches 8.4v (or whatever the battery expects), or until some maximum timeout expires. the time taken to charge - which is just to cover self-discharge over the last week - may be usable an indicator (over time) of 'battery health'.


cheers,
rob   :-)
Edited 2023-03-22 21:33 by robert.rozee