Just a thought – but one I’m putting together as a prelude to adding a controller… what do you think?
So the idea is simple enough – each of these boards is DIRT cheap… the MT3608 boosters are able to output 2 amps. the smaller charging board however is only able to output a little over 0.5 amps – useless for powering, say a Pi3. So – the little board at the bottom charges the battery as long as there is incoming 5v. Similarly as long as there is incoming power, the top booster powers the Pi…. it’s output being 5.8 roughly, dropped by a diode. The middle board, powered by the battery, has a SLIGHTLY lower output voltage and so thanks to the diode arrangement does nothing.
If the power fails, the top board outputs nothing – and so the middle board takes over.
Here’s the thing, if the battery gets well down and the power comes back up, the top board takes over powering the load, the middle board does nothing and so the bottom board can concentrate on charging the battery as quickly as possible.
Not actually TRIED this in practice – hopefully will get a chance over the weekend – any comments before I get the soldering iron out? If you have an old laptop battery, the total cost of this up to now is around £3 or so. The plan would be to stick a Nano in a final version controlling the outputs of those MT3608 boards (they have an enable pin on the chip) and using the analog input to measure battery voltage, ensuring the PI (or whatever) is given the opportunity to shut down when the battery is low – and also (new thought having read what Tesla has done) limiting the battery charge to below full – limiting to 85% apparently does WONDERS for the battery life.
Thoughts? And yes I’ve seen lots of boards out there that do all this, yes they’re expensive – as the cheap ones by and large fail to provide UPS somewhere along the line – usually during recovery from a flat battery. so – I thought, why not DIY… I just happen to have a pal who’s a whiz at PCBs.
Update 15 October
Of course part of the reason for using TWO of these boosters is the inadequacy of the charging circuits based on the common TP4056 – but thanks to Antonio pointing me to a link to a Chinese lady paralleling them up – I then went off on a search and found these… a cheap 3 amp circuit (the only worry is it says in the ad that it is a 4056 circuit but I’m not seeing a 4056) and a slightly more expensive multi-4056 circuit. With this one only then needs a single booster circuit which does not then need outgoing diodes. The multi-4056 circuit seems to be simple enough – all grounds common – all inputs common, all outputs/battery+ commoned. 3 amps output would typically allow for running, say. a PI and providing a reasonable amount of charge to the battery at the same time.
Thankd to this video - https://www.youtube.com/watch?v=uJmQ9W3Yqc8 we have a circuit for multiple 4056 in parallel…
Looking at the spec of the chip it should be able to handle 800ma without breaking a sweat – so 4 of them would give 3.2 amps. I personally would not use those light outputs – I’d take them to the NANO (with internal pullups, bearing in mind the LED outputs are open collector). That would then give the NANO additional information.
According to THIS spec sheet – pin 8 on the TP4056 is an enable pin, normally high. They could be paralleled up and taken to the NANO – which already could control the output (via the enable pin on the MT3608) – ensuring the battery is never too discharged – but now the enable on the 4056 along with the analog input on the NANO could ensure that the battery is never fully charged – and that of course increases battery life. I2c (2 pullup resistors) would allow for programming things like maximum charge voltage, minimum discharge voltage and more – and remaining Nano pins could if needed be used as status LEDs.