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Topic: Simple way to derive 4v + 4v from a 12v supply? (Read 552 times) previous topic - next topic

Robin2

It is easy (using an LM317) to create a voltage that can be used to charge a single LiPo.

I thought (incorrectly) that I could use two LM317s stacked one on the other to create a simple balanced charger for a pair of LiPo cells connected in series. But that does not work because the output of the lower LM317 cannot act as a sink for current from the top one.

I have come across circuits that provide a virtual ground using an LM317 and an LM337 - but they seem to be only capable of dividing the voltage in half - hence 12v would give 2 x 6v and I would need a third regulator to step down the 12v to 8v before halving that. This seems to me to be getting too complicated and it would be easier (although there is a delay) to buy a ready-made 2S LiPo charger.

I am just posting here in case someone is aware of a simple circuit that I can use to give me two stacked regulated voltages from a single 12v supply?

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

bluejets

Hi R,
         Some info on that here.....

http://laserpointerforums.com/f67/will-work-diy-2s-lipo-charger-89998.html

cheers Jorgo

Wawa

Seems dangerous.
If the lower battery is still <4.2volt, then the top battery could get more than 4.2volt.
Leo..





bluejets

Wondered if something along these lines would suit you.

http://www.ebay.com.au/itm/Gpower-G3220-LiPo-Life-2s-3s-0-5A-1-2A-2A-and-Voltage-Balance-Protection-Charger-/311492908230?hash=item48866c94c6:g:4fQAAOSwu-BWPFKw

Robin2

Thanks. The link in Reply #1 may have a solution using a regulator and an OpAmp - I need to see what parts I have in my box of bits.

I am hoping to avoid buying a charger (just being stubborn) - but I realize I may have no choice, longer term.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

Robin2

Update.

The circuit from page 2 of the link in Reply #1 seems to work nicely. I am just using an LM358 OpAmp because it is what I had in the box.

I only want to charge a pair of either 80mAh or 240mAh LiPos. I don't need fast charging and they won't be deeply discharged.

Thanks again.

...R

Two or three hours spent thinking and reading documentation solves most programming problems.

larryd

@Robin2
Quote
just being stubborn
This may help:
Planning an implimenting






Just kidding ;)
Buy a ready made solution so your house insurance is still valid.


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mrburnette

#7
Sep 01, 2016, 10:17 pm Last Edit: Sep 01, 2016, 10:24 pm by mrburnette
I have had great success over the past couple of years using LiFePO4 cells .... gotten rather inexpensive due to their extensive use in evening lights for home gardens and other solar tidbits.

Nice about LiFePO4 is they can be float-charged on 3.3 to about 80% - 90% or if the hardware can deal with 3.45V then very nearly 100%.  I just use an under $1 DC-DC buck, set it to around 3.4V on the output, hook up the ESP8266 or STM32F103 board, and parallel the battery.

LiFePO4 nevers has a cycle chemistry that produces lithium metal.  The cells can toleration abuse in both discharge and charging.  They are not expensive IMO:

You can fine-tune your DC-DC buck output voltage to lower the trickle to around 1 to 5mA when the cell is fully charged and the device is OFF.  With the device ON, if the batteries are supplying more than a few mA, then you have a "sag' in your wiring from the DC-DC to the batteries.  Size the DC-DC to handle the full project plus whatever 1C of the battery capacity is rated.

Do properly fuse the batteries.  With the DC-DC not having an external supply, the battery should be switched to avoid the circuitry running the cell down below about 2V.  No diode is required from the DC-DC since the module has its own output side diodes.  The output caps on the DC-DC also parallel the battery, so you get the benefit of almost no switch-on battery sag.

Ray

bluejets

@Robin2
This may help:
Planning an implimenting

. Currently working my way through this in the hope of making my new project a goer. No doubt will have a query or two along the way .....thanks Robin2

Robin2

Buy a ready made solution so your house insurance is still valid.
First, it won't be used unattended.

Second, by setting a lower-than-usual voltage you can't easily overcharge a LiPo. The charge current falls off to zero.

I want a simple system that I can drive a model train against so that it will pick up some charge for a minute or 10 and then go off again.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

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