3.7V LiPo parallel combination to 12V DC 2.5A circuit

[abhishek3008]

I am trying to drive a Netgear Wifi router by a parallel combination of 3.7V LiPo (950mAh) cell phone batteries. I want them to run for at least 3 hours on full charge. According to my calculations Ill need around 10 batteries in parallel to achieve this power capacity.

Wifi router input power is rated at 12V, 2.5A.

I am planning to use a MAXIM MAX668 IC for the step-up circuit. I found a reference design that can output upto 2.3A.
I have three questions.
1) In the reference circuit schematic I have attached, I dont understand the (3x) at C1 [47uF , 6V, (3x)] at the input. Does it mean I connect 3, 47uF caps. in parallel? Same goes for the (4x) at C5 near the output.


2) Do you think this design will be able to power the router rated at 2.5A without problems?

3) Is there a better design/way to do this?



Thanks a lot

 

[sky_123]

A far better way is to use a battery with technology people are more familiar with (e.g. Lead Acid or NiCd).
Then you have to worry less about what happens if you charge the cells unevenly, or try to discharge too
much or at too high a rate.
There is a good example here, i.e. a
12V battery.
Then you can get a DC-DC converter (e.g. TracoPower) that will supply 12V from 10-40V for example, so you could use
that, or assemble a circuit. An example IC would be Texas Instruments LM5118, there is a circuit diagram for 12V 3A output
in the data sheet.
BTW the router may not use 2.5A, that is probably a worst case, max rating.
And better still, by going this route, you can even power the router in your car if you ever needed to.

 

[abhishek3008]

I really appreciate the time and effort you made for the reply...thanks!
However, there is one major motivation for using LiPo batteries - we have at least 50 brand new ones lying in our lab. We want to make use of them.

It would be really helpful if someone can let me know what the 3X and 4X in the circuit schematic at C1 and C5 mean....
I know using a 12V battery pack will be much easier, etc. but the sole purpose of this project is to use those LiPo batteries.

Actually there is a similar project documented here: http://punish3r.hackhut.com/2011/03/27/10-2-ah-lipo-pack/
unfortunately, the project does not talk about the specifications of the boost converter. He just says its an engineering sample.

 

[mtwieg]

Yeah 2.5A at 12V sounds like too much power for a router. That's probably just a max rating for the wall wart. I would actually measure its average consumption during use. It's probably more like 1A.

 

[sky_123]

It just means use several capacitor in parallel. However, bear in mind, that if (say) the router takes 12V 1A current consumption, then 3.7v batteries need to supply at least around 3.5A or more,
depending on DC converter efficiency. I don't know if the batteries are designed for that kind of level (and I'm no expert on LiPo). Furthermore, you have the issue of how to charge them, and if it
is ok to parallel them if they are dissimilar. Personally I wouldn't do it, and I wouldn't trust it to be safe to use unless you're very familiar with LiPo. If you've ever seen exploding components, then
you'll know it's never pretty. And the MAX668 circuit doesn't monitor the temperature of the cells or anything.
The 12v lead acid is about $30, probably just as compact, and will do what you need, and will be safe.

 

[bobstay]

The 12v lead acid is about $30, probably just as compact, and will do what you need, and will be safe.

If you do use a 12V lead acid, don't do what I did! I just connected the lead-acid in parallel with the router's 12V power supply (because it was a 12V battery), expecting that the router power supply would slowly charge it, and when the power died, the battery would discharge and power the router.

Unfortunately, this means the lead-acid floats at 12V, rather than the full-charge float voltage which should be 13.8V. This meant that the battery was barely charged at all, and worse still, got sulphated quite quickly, making it useless!

If you do want a "UPS-style" solution with a lead-acid battery, you will need some kind of proper float charger for it, and maybe an LDO regulator (or buck-boost) if your router can't cope with 13.8V at the input.

And if you do use the li-po cells, you need to be even more careful charging them!

 

[pia32]

There are high capacity high discharge batteries intended for use in RC that might be suitable
**broken link removed**