3

I'm more of a software guy, so I need your help to figure out the power issue with my Arduino.

What I would like to do is using a GSM/GPS unit (Ai-Thinker A7) with Arduino Nano as a car tracking system. I have USB cables in the car, so I can plug it into my Arduino and power both while I'm driving. To power the unit while I'm not driving, I thought using rechargeable batteries would be a good idea. I'm planning to use GPS via SMS activation, so I hope fully charged batteries will be enough to power it for at least a week. But I'm confused about a couple of things.

Rechargable batteries are 1.2V. So technically I can use 5 of them and power my Arduino and the GPS unit (which requires 5V), right? However, for a similar unit at Adafruit they say that a LiPo battery is a must. Well, I think it is because GPS unit draws more power while using GPS and stuff. So, in this case are AA batteries out of the question? Will I fry anything if I build the system as follows?

Circuit

If AA batteries happen to be fine, do I need another piece in my design, such as this one?

At last, I have no idea if I need any diode, capacitor, resistance or sth else for this system. I would appreciate further comments about my design.

|improve this question
  • 1
    you need to be able to supply 2 amps. AA batteries can supply that for short periods. if find you needed more amps, you can add more cells in parallel instead of series to increase available amps. you might want to consider a buck/boost converter than can turn any (reasonable) voltage into another voltage. – dandavis Mar 23 '17 at 6:33
  • 1
    If you are using NiMH batteries watch out for the self discharge, they can lose almost 50% of their charge in a week. It increases with temperature and in the summer cars get hot, you could easily find that your batteries don't last a week in the car even with nothing connected to them. – Andrew Mar 23 '17 at 9:08
  • 1
    @SertalpBilal Li-Ion have very good self discharge characteristics but long periods of high temperatures can ruin their maximum capacity and they are a lot more complex to charge / make safe. Your best bet for a simple solution may be the low self discharge NiMH cells you can get (also sold as "pre-charged" rechargables). They cost a lot more than standard rechargeables but would avoid the issue. – Andrew Mar 23 '17 at 14:02
  • 1
    @SertalpBilal yes. – Andrew Mar 23 '17 at 14:10
  • 1
    The voltage regulator on the Nano has a dropout of 1.25V which means you need 6.25V on the input. That works out as 1.25V per cell if you have 5, it depends on loading and temperature but for low power applications most batteries hit that at about 50% charge. Probably best to go with 6 cells. On the plus side that should make finding a battery holder easier, 6 cells is more standard. Other than that I can't see any obvious issues. – Andrew Mar 23 '17 at 14:41
2

I suggest using 6 or 8 AA's (eg 6 alkalines or 8 NiMH) and a MP1584EN DC ­DC Power Stepdown Buck Converter (about a dollar on ebay, 1.5 to 3 times that on Amazon). Note, the hole pattern does not match up with breadboard spacing: enter image description here

The batteries would tie in via a couple of diodes (preferably Schottky), as shown below. When the 12 V supply is off, the converter would be powered by 9.6 V from eight rechargeable AA's. When the 12 V supply is on, no current is drawn from the AA's. The converter output should be adjusted to 5V before being connected to the Nano or other devices it powers.

Using a buck regulator has two advantages vs a linear regulator: battery current is lower (allowing the battery pack to be more efficient as it discharges) and less energy is wasted as heat.

Note, it might make sense to remove the power LED from the Nano, to save 3 to 5 mA of current.

schematic

simulate this circuit – Schematic created using CircuitLab

|improve this answer
  • Thanks for the detailed response. A few questions: Is there any advantage of using 6/8 AA batteries with a step down converter compared to using a LiPo (3.7V) with a step up converter? I worry that the device will get heavier/bulkier with that many batteries. Another question is, can I recharge batteries by plugging my Arduino into the USB in this case? It seems diodes will prevent the current into charging direction. – Sertalp Bilal Mar 23 '17 at 13:59
  • Not so sure if this is the one I'm looking for, but do you think something like this would work? – Sertalp Bilal Mar 23 '17 at 14:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.