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How to limit the max voltage in a charging circuit?
3

How to limit the max voltage in a charging circuit?

How to limit the max voltage in a charging circuit?

(OP)
I'm looking for a simply way to limit the max charging voltage for an existing charging circuit.

I've heard this can be done with diodes, but am a bit clueless as to how to calculate the rating of the diodes, and/or whether this would be the ideal solution, since I don't want to cause any damage to the original charging circuit.

I have two applications in mind at present:

1. Limit the max charge of a USB-C charger for a laptop, so that it charges to around 80%
[many laptop manufacturers offer this as an option to increase life of lithium batteries, unfortunately mine does not]

2. Limit the max charge of a tool battery charger to 28.8v instead of 30v.
[it's a "24v" NiMh/NiCd charger so really should only charge to 28.8v, but has been designed to "fast charge" - in other words to dramatically reduce the lifespan of batteries, thus requiring regular replacement of batteries that should last 3 times longer - a clever way to guarantee a roaring trade in "consumables"]

Does anyone have any ideas, or able to point me in the direction of some circuit designs?

RE: How to limit the max voltage in a charging circuit?

Unfortunately with USB-C the actual voltage you get for power is between 5V and 20V and is determined by communication between the USB-C power device and the device being powered.

On top of that, something like a laptop is likely to have more voltage controlling circuits. It may receive a voltage of 5 to 20V on the USB-C input and may internally boost the voltage to the voltage suitable to charge the battery. Attempting to drop the input voltage slightly may only make the boost converter work a litter harder but the batteries will still get the voltage required to charge. You may only succeed in making the laptop take a little longer to charge to 100%.

Best way to extend battery life of lithium batteries is never to run them to full discharge.

RE: How to limit the max voltage in a charging circuit?

You should seriously stop worrying about it. I'm typing on a Samsung Laptop I put in service in 2012. I keep it charged on the charger 24/7. It stays between 97% and 100% charged. If I take it in the field it still runs more than 2.5 hours. New it ran 3 hours. Worrying about battery life on anything rechargeable is a fools errand. Life's way to short and complicated for a human to spend any of it babysitting a hundred bucks of batteries.

My EE buddy constantly perseverates over trying to keep his Nissan Leaf at 80% or less. The result is extra life stress and his inability to actually go some places because of the range limitation caused by his idiocy.

Your laptop will either work great for many years or one of the cells will check-out early completely unrelated to the charging level, so don't worry about it - be happy. :)

Keith Cress
kcress - http://www.flaminsystems.com

RE: How to limit the max voltage in a charging circuit?

(OP)
Thank you both... I've seen the light, yes life is too short to worry about laptop batteries!

On the other hand, the tool batteries really are an issue, they start to give diminishing returns within a few months and past 12 months are fairly useless.

So.. still open to suggestions.

RE: How to limit the max voltage in a charging circuit?

You could possibly buy a wimpier charger for the batteries. The Ryobi system comes with a beefy charger, usually, but they also have a much tinier, and presumably, wimpier charger

TTFN (ta ta for now)
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RE: How to limit the max voltage in a charging circuit?

Thanks for recognizing the laptop situation.

About your power tools... What brand? Some brands have very poor quality batteries and that's what happens - lots of battery life problems.

You mention NiCad that seems to date your tools as.... ancient? I will say that NiCd in power tools is very very hard to maintain their capacity. NiCd are really very lame with their tragic 'memory' issue. All my NiCd based products crapped out in short order. Even a $5K oscilloscope with NiCds, the battery lasted barely 2 years (if that).

NiMH are just exactly as bad as NiCd with the same chronic memory issue. Nothing NiMH ever lasted long either. I call you trying to do anything to extend NiCd or NiMH battery system lives as being next to impossible and extremely frustrating.

This would be my take: Milwaukee batteries are considered to be the best at the moment. Ryobi the worst batteries, followed by better Makita and better yet Dewalt. Dewalts were crap but have improved remarkably in recent years. I have seen many adapters lately to adapt Milwaukee battery packs to everything else. You plug a Milwaukee pack into an adapter and then plug that into the 'other' brand tool.

I've 3D printed things that you slide onto a Milwaukee M18 pack. Works well. Get out of the NiCd/NiMH nightmare and into the much higher performance and better maintained LiION.

Here's just a random example of a Milwaulkee to Ryobi 18V adapter. Amazon has a bunch.

Keith Cress
kcress - http://www.flaminsystems.com

RE: How to limit the max voltage in a charging circuit?

(OP)
Now we get to heart of the issue with the tools!
Yes they're ancient - about 2006/2007, but they're Bosch Pro and very well made - drill, jigsaw and circular saw.
I'm looking into upgrading to Lithium Ion, but here's the thing: the tools are 24v
so... 3.7 x 7 would give me a nominal operating voltage of 25.9 and i'd need a max charge voltage of 29.4

in practice I would use 2 x 7 1500mah batteries to give me a 3ah battery

The only problem (and the reason for this post) is that I DO NOT want to over-charge the Li-ion batteries.

Like I said my existing charger will go up to 30v, even though the batteries it was designed for shouldn't be charged beyond 28.8v

I also need to check the max current of the charger to make sure it wont deliver more than the lithiums can handle

RE: How to limit the max voltage in a charging circuit?

There is lots of information about charging li-ion cells - simple constant voltage chargers are not suitable.

RE: How to limit the max voltage in a charging circuit?

I agree with Dave. You always need a managed battery system for Li batteries.

Keith Cress
kcress - http://www.flaminsystems.com

RE: How to limit the max voltage in a charging circuit?

Use parallell Zener diode and a series source Resistor
Its voltage (refers to the reverse breakdown voltage) ranges of 2.4 V to about 200 V (some special one can be up to 1 kV). Most the surface-mounted device (SMD) has maximum about 47 V
Simply that value is guarding/supplying the intended Voltge parallel to it with supplied current is about Vs / ( the series source Resistor + Battery internal Resistance )

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