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'[EE] Batteries in parallel'
2008\02\19@120245 by Ariel Rocholl

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I would like to connect two batteries in parallel to service a RF
receiver. Both are Lipo batteries but battery A is high capacity and
main battery whereas battery B is reserve low capacity battery.

The average consumption of the RF receiver is about 2A, battery A is
5A/h and battery B is 1.5A/h.

I do not see an easy way to:

1) have battery A depleted then (and only then)
2) switch to battery B till it is depleted too.

I would assume a large capacitor plus a very fast switching transistor
(IGBT?) may help to make it transparent to the load to a certain
extent.

But perhaps anyone knows a better way to do this. Any help really
appreciated, I wouldn't like to reinvent the wheel but, except for low
current ICs in Maxim, I didn't find anything else doing this already.

TIA

--
Ariel Rocholl
Madrid, Spain

2008\02\19@121811 by M. Adam Davis

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Cheap and easy:

Use a diode to pass the current from the smaller battery.  It won't
contribute (much) to the load until the main battery has a voltage
less than the backup battery minus the diode voltage drop.

-Adam

On 2/19/08, Ariel Rocholl <spam_OUTforosTakeThisOuTspamarocholl.com> wrote:
{Quote hidden}

> -

2008\02\19@123006 by Dr Skip

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You could use FETs or even low power relays. Don't do any fancy switching,
since you can overlap the batteries for a small instant for the switching.

Use either a comparator or a lm3914 (IIRC) as voltage level detector. Control
the relays as inverse of each other and use a 555 timer to overlap the
batteries for some small time (1 sec?). You could probably just put a few
hundred uF cap in parallel with the first relay so it stays latched a second or
two after the switching. QED. ;)


Ariel Rocholl wrote:
{Quote hidden}

2008\02\19@124603 by Bob Axtell

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You cannot do this because you cannot charge either battery correctly.
You MIGHT be able to do something with a latching relay, but you will need
to control it and sort out the  charging circuit.

It would be easier if you had two identical batteries.

--Bob A

Ariel Rocholl wrote:
{Quote hidden}

2008\02\19@131725 by Ariel Rocholl

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I should have clarified that battery charge process is done
separatedly (in other words, batteries are plugged in completely
charged already).

2008/2/19, Bob Axtell <.....engineerKILLspamspam@spam@cotse.net>:
{Quote hidden}

> -

2008\02\19@132245 by Ariel Rocholl

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This sounds like a sensible approach. Difference between a full and a
depleted Lipo is just 1.2v max per cell, so I think 1 sec overlap
shouldn't hurt the reserve battery too much. I'll do some testing to
understand max current being taken from full battery when overlapped
to confirm it is still on product margins (10C=15A for this reserve
battery).

Thanks

2008/2/19, Dr Skip <drskipspamKILLspamgmail.com>:
{Quote hidden}

> -

2008\02\19@132405 by Ariel Rocholl

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I should have clarified that battery charge process is done
separatedly (in other words, batteries are plugged in completely
charged already).

2008/2/19, Bob Axtell <.....engineerKILLspamspam.....cotse.net>:
{Quote hidden}

> -

2008\02\19@132732 by Ariel Rocholl

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face
This sounds like a sensible approach. Difference between a full and a
depleted Lipo is just 1.2v max per cell, so I think 1 sec overlap
shouldn't hurt the reserve battery too much. I'll do some testing to
understand max current being taken from full battery when overlapped
to confirm it is still on product margins (10C=15A for this reserve
battery).

Thanks


2008/2/19, Dr Skip <EraseMEdrskipspam_OUTspamTakeThisOuTgmail.com>:
{Quote hidden}

> -

2008\02\19@144414 by David VanHorn

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On Feb 19, 2008 12:29 PM, Dr Skip <drskipspamspam_OUTgmail.com> wrote:
> You could use FETs or even low power relays. Don't do any fancy switching,
> since you can overlap the batteries for a small instant for the switching.

That's not as easy as you think, there's a body diode that will allow
both batteries to feed to each other.
google batfet.

2008\02\19@151341 by Dr Skip

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I believe you can get them without, but if not, relays. The added benefit is
real power disconnect for an off state as well, no ESD issues, and consumption
is a small fraction of load consumption anyway for micro types. You also get
the delay with just a cap.


David VanHorn wrote:
> On Feb 19, 2008 12:29 PM, Dr Skip <@spam@drskipKILLspamspamgmail.com> wrote:
>
> That's not as easy as you think, there's a body diode that will allow
> both batteries to feed to each other.
> google batfet.

2008\02\19@160011 by David VanHorn

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On Feb 19, 2008 3:12 PM, Dr Skip <KILLspamdrskipKILLspamspamgmail.com> wrote:
> I believe you can get them without, but if not, relays. The added benefit is
> real power disconnect for an off state as well, no ESD issues, and consumption
> is a small fraction of load consumption anyway for micro types. You also get
> the delay with just a cap.

That's why they do BatFets, but IIRC they are actually significantly
more complicated internally than just a couple of fets.   Relays suck
power, and are huge, but do pretty well at the switching, I agree.

2008\02\19@173414 by Rich

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I would think that a small resistor, say 50 ohms, in series with each
battery being charged work?

{Original Message removed}

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