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which Rechargable Battery i should use for powering up GSM Module SIM800.

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i am looking for a rechargeable battery for my device which using SIM800 for communication. i want to get a notification in case of power failure thats why i am looking for battery which can provide the required power in absence of mains power. Because my device remains ON for 24X7 so i am looking for rechargeable battery so that once i fit the battery it can works for months.
SIM800 voltage requirement =4v
current 300mA
Burst current 2A while message firing.
 

Nigel Goodwin

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18650 Li-Ion, standard Li-Ion batteries - if you're only using it with a semi-permanent mains supply, you MUST ensure that it's charging requirements are met (you can't just leave it on trickle charge, as it's likely to explode and set on fire).

I would also suggest a 1 Farad supercap across the supply near the chip, as the high current pulses can occasionally dip the supply enough to reset the SIM800.

As I'm using battery power only I switch the power to the SIM800 using a dual FET switch, so as to minimise current when not transmitting. On a single 18650 I've recently run it until the battery was too low (for testing purposes), it lasted just over 4 months transmitting every 8 hours, a total of 378 uploads. I've recharged it, and put it back running again - and it's not even a new 18650, it was out of an old battery pack we replaced.


The actual ones 'out in the field' have two 18650's in parallel, and only transmit weekly, so apart from self-discharge they should last a LONG time - we're waiting to see what happens (among the required data they send, they also report the battery voltage).

For long battery life shut down everything that's not needed, and just run a 32KHz clock that wakes it up every second - mine wakes with signal input as well, but for checking mains there's no need - just use the 1 second clock trigger and check if the mains is present, and react accordingly.
 
In my case i will only use battery when power is not available, so in that case i think it will run for more than 4 months.
 

Nigel Goodwin

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Most Helpful Member
In my case i will only use battery when power is not available, so in that case i think it will run for more than 4 months.
It should run a LOT longer than 4 months, even on the battery alone, as presumably you won't be transmitting every 8 hours (hopefully!).

How often are you likely to lose power?, and do you want messages both when it goes OFF, and also when it comes back ON. The 378 messages I received from one 18650 is a lot of messages really.
 
It should run a LOT longer than 4 months, even on the battery alone, as presumably you won't be transmitting every 8 hours (hopefully!).

How often are you likely to lose power?, and do you want messages both when it goes OFF, and also when it comes back ON. The 378 messages I received from one 18650 is a lot of messages really.[/QUOTE
Maybe 10 times a day, yes i need msg both in ON and OFF condition.
what do you think about a 741 opam and QN222 transistor based charging circuit?
i have attached a image of battery is it the correct one?
 

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Nigel Goodwin

Super Moderator
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Yes, those batteries should be fine.

You don't make Li-Ion chargers with a 741 and a QN222 - either do it in software (plus the charging hardware) or simply buy a Li-Ion charger board, these are commonplace and cheap, and take a 5V supply (usually intended for USB).

If the mains is that bad, I would suggest using two (or more) 18650's in parallel!.
 

crutschow

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What about using a standard UPS supply from the mains?
 
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