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I'm trying to power a GSM modem from 2 AA NiMH batteries in series, and wonder what my options are. Main consideration is that it needs to be USB rechargeable, so I only have 2 batteries to work with. The modem can work from 3.3V to 4.6V and draw 2A of current bursts. Can anyone recommend a boost converter that will suffice here?

So I have 2.4V input, need 3.6V out @ 2A, max ripple is 400mV

Will a TPS61230 do the trick? Its Vin(min) is 2.3V though, so once the batteries loose a little charge will their voltage be just too low?

Another option is 3 x AA alkaline, but their internal resistance is a bit high (approx 80mOhm). However 3 alkalines is 4.5V, very close to the upper limit. If the batts were even at 1.6V, this would run the risk of a fried module. Maybe I could drop the voltage across a diode MBRS340 or similar (4A, VF 500mV).

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  • \$\begingroup\$ New alkaline battery can have 1.65V voltage (3x1.65 = 4.95V). \$\endgroup\$ – Kamil Dec 16 '14 at 20:14
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    \$\begingroup\$ @TJK are you sure you cannot use a lithium battery? There are a lot more chargers available, and they have power path features which are very system friendly. Recharging NiMH in circuit is always a problem. I have not seen NiMH chargers which incorporate power path switching (which isolates the batteries from the load while charging). Without this power path feature, it is hard to charge the batteries at a constant current, and impossible to use dV/dt charge termination because the variable system current will cause small fluctuations and trip the dV/dt detector. \$\endgroup\$ – mkeith Dec 18 '14 at 18:48
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I had similar problem with driving power LEDs (about 3V) from two alkaline or NiMH.

What LED driver IC options do I have to build a two-cell driver for a 3 W LED?

I is hard to find ANY converter that works with input voltage near or below 2V (empty NiMH or alkaline voltage is about 0.8-0.9V). I found LT1618 with minimum input voltage 1.8V and maximum current 1A, but it is hard to buy it where I live.

I guess that if you connect big enough capacitor (tantalum?) at converter output - it should be enough for 2A current bursts.

However I think you should reconsider your battery choice if you can, because:

  • you may be unable to drain enough current from partially discharged alkalines to supply modem
  • using higher voltage (3xAA/AAA or single Li-Ion) allows you to use many typical DC/DC converters
  • when you use higher voltage - high converter efficiency will be easier to archieve
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The three alkaline AA cells will probably not work, due to the ESR of the batteries as you surmised.

While a GSM module draws 2A, it is only for a brief period, around 575 µS every 4.6 ms.

An AA alkaline battery can output 2A for a brief period, but because of the internal resistance of 80 mΩ you will lose almost 1/2 volt (.08Ω * 3 cells * 2A) while it is outputting the 2A.

This means you will have a maximum voltage available of 4.5V - 0.5V - 0.5V = 3.5v

where the first 0.5V is the forward voltage of the diode, and the second is the drop due to the ESR of the batteries. This leaves no headroom at all for discharge of the batteries.

Even with a Schottky diode with a slightly less V\$_F\$, like the MBRS410LT3 (0.25V at 2A), that doesn't help very much.

You could try adding add a a large tantalum capacitor in front of the cell module, but that also won't help that much since it will have a similar ESR.

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So it looks like the boost IC you chose, TPS61230, is a better option, since you can keep the output voltage of the boost IC constant. However as you point out, V\$_{IN}\$ minimum is a little high. The LTC3425 has about identical specifications, but has a V\$_{IN}\$ minimum of 0.5V.

If you want to use AA NiMH batteries, you should be able to use either two or three cells since the fully charged voltage per NiMH cell is 1.2V, and the total will always be below the maximum of the cell module. Be sure to use an IC specifically designed to charge NiMH. The LTC4060 can charge from 1 to 4 NiMH calls in series.

Now you can run the output voltage closer to 4.6v. I would add a tantalum capacitor (or capacitors) of at least 200 µF and as low ESR as possible to smooth out the transient from the 2A draw. If you have trouble finding tantalums with a 5V rating and low ESR, you could lower the output of the boost circuit to 3.7V and use one or two 4V capacitors like this one. Turns out 3.7V is what cell modules are really designed to work at, since that is the nominal voltage of a single-cell Li Ion battery (which can go up to 4.2V or so when fully charged).

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Another option would be to go with with 4 alkaline batteries (4*1.65V or 9.90V new, according to a comment to the OP, and 4*1.5V, or 6V nominal), and use a buck-boost regulator that can handle 10V of input like the ISL85403. That way they could drop to 0.78V apiece (2.5V / 4 + 0.16V) and still operate (where the 0.16V is the drop due to ESR). (You probably can't really run with battery voltages quite this low, since if I remember correctly the voltage of a alkaline battery drops rapidly once the cells go below about one volt.)

With four cells, you will lose a total 0.64V due to the ESR of each battery, but if you keep the output of the buck-boost at 4.6v, that should not be a problem. I would still add a tantalum capacitor (or capacitors) of at least 200 µF and as low ESR as possible to smooth out the transient from the 2A draw. Once again if you have trouble finding tantalums with a 5V rating and low ESR, you could lower the output of the boost circuit to 3.7V and use one or two 4V capacitors.

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  • \$\begingroup\$ I just want to add that Eneloop NiMH cells have very low internal resistance (25mΩ in near-empty state). According to datasheets and discharge curves - looks like you can constantly pull 4A from AA or 1.6A from AAA cell. \$\endgroup\$ – Kamil Dec 16 '14 at 20:34
  • \$\begingroup\$ thanks @tcrosley, however i dont want to use more than 2 nimh as im restricted to charging via usb. All info i have is that more than 2 batteries needs more than 5v to charge it. \$\endgroup\$ – TJK Dec 16 '14 at 23:08
  • \$\begingroup\$ @TJK my answer said "you should be able to use either two or three cells" -- the charger IC I referred to can handle either. The schematic on the lower left of the first page of the datasheet shows a 2 cell, 2A configuration. \$\endgroup\$ – tcrosley Dec 16 '14 at 23:40
  • \$\begingroup\$ The LTC4060 will not terminate charge properly if the system load is in parallel with the batteries. It relies on dV/dt sensing for charge termination, and any noise on VBATT due to variable system current will interfere with it. \$\endgroup\$ – mkeith Dec 18 '14 at 18:55
  • \$\begingroup\$ TI has a bunch of boost regulators designed for one or two cell NiMH and alkaline systems. Here is one that might work for you: ti.com/product/tps61031 \$\endgroup\$ – mkeith Dec 18 '14 at 18:55

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