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Author Topic: I'm looking for a high current programmable battery charger.  (Read 222 times)

KI6R

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I'm looking for a high current programmable battery charger.
« on: August 01, 2021, 11:48:19 AM »

I have three Toyota Prius NiMH battery packs which I have re-wired into three 14.4V 91AH battery banks.  My plan is to parallel all three banks into a single regulated 13V output.  All of the battery paralleling has been done using Schottky diodes.  The total battery capacity will be about 273AH at 14.4V.  Figuring out how to charge this much NiMH is the hard part.  NiMH batteries require a current limited source for charging.  I'd like to charge at a current limited 0.1C to .4C rate, 27.3A to 81A.   The charger voltage would need to be at least 20V.  Does anyone know where I can find a programmable current limited charger that can do this?  Perhaps a 24V programmable 50A charger?  Lastly, if it is a switch-mode charger I'm worried about HF RFI.  If I can't buy a charger at reasonable cost I will be designing and building one.  Thanks for any suggestions.  Ed, KI6R, Tucson.
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WB6BYU

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Re: I'm looking for a high current programmable battery charger.
« Reply #1 on: August 01, 2021, 03:38:47 PM »

Might be easier to use a separate 10A charger for each of the
3 units.  Certainly easier to carry around!

KI6R

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Re: I'm looking for a high current programmable battery charger.
« Reply #2 on: August 01, 2021, 04:37:00 PM »

Three chargers is not a bad idea.  I already have the diode isolation.  Thanks, I'll consider it.
73
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WB6BYU

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Re: I'm looking for a high current programmable battery charger.
« Reply #3 on: August 01, 2021, 06:51:36 PM »

Some of the standard 13.8V power supplies might have a high
enough voltage out of the rectifier to provide a good starting
point for the current limiter.  Otherwise a stock 24V supply
would work.

There was a simple NiCad battery charger  years ago in ham
radio
magazine that had both voltage and current limiting.
I built one for my HT and kept the same battery on charge for
8 years (until the supply powering it failed, and the battery
went dead).  The heart of the regulator was something like
one transistor and a pair of zener diodes (although I think
I used LEDs for lower minimum voltage drop, which also
provided status indicators).

LA9XNA

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Re: I'm looking for a high current programmable battery charger.
« Reply #4 on: August 02, 2021, 01:39:54 AM »

This is basically just a constant current source with voltage limitation on the input.
The regulated voltage can be a regular switching power supply.
To make the current limiter you can in theory build it with a Zener diode a resistor and a BJT. But because this is high current you have to beef everything up.
If you are using the 2n3055 you will need about 10 of the transistors (120W/24V= 5A).
hfe is in the range of 20-70 so the total base current must be somewhere between 2,5A and 0,8A.
Shunt resistor must be 0,014ohm and 35W.
The voltage reference must probably be buildt in several stages to be able to handle the current for the 2n3055 transistors. You can get away with on 100mA, on 1A and a 2n3055 for  the voltage reference or possible a LM317 and a 2N3055.
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K6AER

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Re: I'm looking for a high current programmable battery charger.
« Reply #5 on: August 02, 2021, 07:24:17 AM »

How expensive was the LiPo battery packs and what is their size? Just curious.

By isolating the battery packs with diodes you will need three chargers. You are much better to tie them all together. Current only flows one way on a diode.  As for a charger I would use the IOTA-45 unit. It has a float charge of 14.1 volts and a output  of 45 amps. RFI is non existent, I use their IOTA-90 on my station.

What is it you are trying to do with your station?

Is you requirement of DC power only for a 100 watt transceiver.

Will there be other accessories?

Long term power is best handled in emergency situations with a generator or the more expensive solar array. It would help to have the big piture.
« Last Edit: August 02, 2021, 07:28:07 AM by K6AER »
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W9IQ

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Re: I'm looking for a high current programmable battery charger.
« Reply #6 on: August 02, 2021, 07:53:00 AM »

If you plan to charge the NiMH cells at 0.1 to 0.4C, then you will need a more sophisticated charge management controller than simply current limiting.

If you drop your charge rate to 0.05 C, plus a cutoff timer, then NiMH chemistry is fairly tolerant of a constant current charge. This is worth considering if you do not require rapid recharging.

Using a 12 or 13 volt regulated supply will not successfully recharge the cells and allow you to realize their ratings.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KI6R

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Re: I'm looking for a high current programmable battery charger.
« Reply #7 on: August 02, 2021, 07:36:58 PM »

LA9XNA: thank you for the comments.  I think you are correct on the current limiter design.  Something much like your description may be what I build.
K6AER:  The packs are NiMH and their cost was about 2 weeks of design/construction work creating a one-of-a-kind high voltage Prius battery pack load box and charger for a auto scrap shop that resold batteries.  I'd like to use something like the IOTA but I need at least 18V in order to charge the batteries.  I have already built a 25A voltage regulator for the bank which will run a 100W HF station and accessories.  I will feed a 500W solid state amp directly from the batteries.  I may end up with solar also charging the packs at some later date.
W9IQ:  These NiMH batteries are a little unique.  Toyota ran their state of charge within a narrow window of 40-60%.  Large currents were constantly going in and out of the cells, but only within the narrow SOC window.  The rumor is that this enabled extreme battery longevity.  What I heard is no one really knows how long these batteries can last if operated in this window.  Most Prius batteries fail due to leaking post terminals.  0.05C or a little higher with a timer is my goal.
   I found two 9VDC 37A switchmode supplies today at a local surplus store.  They have fully isolated outputs and adjustable voltage, over-voltage and current limit.  I'm going to series the supplies and dial them up to about 19V.  I'll find out if I need an external current limiter.  Thanks for all the comments and encouragement.  73, Ed, KI6R
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