Lithium batteries questions

So work is continuing on the glow fans. I find myself wanting to run the next version off lithium batteries, but I’m a bit confused. Ok, overcharging them is bad, so I’ve a design for a proper charging circuit, based around the Linear LTC1734. And that’s all well and good.

Discharging them is another matter, with dire warnings for letting the battery voltage get too low. However, here is where I get confussed. Surely whatever circuitry that is responsible for monitoring the shutdown is going to draw a load? Is it the case that so long as the load is low, then the battery won’t be damaged? How low?

Current plan for the low battery behaviour is to have the Picaxe wake up once every umm.. few seconds, check that charging hasn’t started, make sure that everything else is off, and then snooze again for a while. The Picaxe gets its 5V from a charge pump converter (a MAX1595) with a 0.2 mA draw when the Picaxe isn’t drawing power.

(I think this is the main power draw when off. There’s other gubbins – the other step-up converter to power the LEDs draws only 5 uA when shutdown, but needs a pull-down resistor to keep it shutdown, which will have it’s own current needs. But I think all that should be much smaller than the 200 uA from the charge pump.)

Is that all I need to do? Just have the Picaxe mostly off and a 0.2 mA draw running on the battery? Or are there better ways to do this? And yes, some lithium batteries come with under-voltage protection built in. If under-voltage is so bad, why don’t they all? And why can’t I find an undervoltage protection chip for just one lithium cell, instead of the 2-10 that TI offer.

Brain all hurty now.

5 thoughts on “Lithium batteries questions”

    1. This one?

      “The above circuit does it. PB1 is a pushbutton. SWSENSE and PSUON are input and output to the CPU (an Atmel 644, but could be anything). the CPU and everything else are powered by VCC, which is an output from this circuit.

      When powered up from cold, the MAX710 is held in standby by R18 & R19 pulling /STBY low. Pushing PB1 pulls this pin high and starts the MAX710. The CPU then boots and sets PSUON as an output and drives it high. This (due to the diodes D12 & D5) holds the PSU on, even when PB1 is released.

      The CPU can read PB1 any time using SWSENSE. Because PS (internal power supply of the MAX710) can reach a value higher than Vcc (exactly what is undocumented by Maxim) this is clamped to below Vcc to avoid stressing the CPU. SWSENSE will be low if PB1 is open and high when it’s closed – thus allowing this input to be used by the application.

      When the application detects that PB1 has been down for some time (like 5s) it will drop PSUON and enter a loop. When PB1 is released, /STBY will be low and the PSU will shutdown. Hence providing switch off. From this state, switching back on happens as above.”

      I never got my head around understanding how that works, possibly coz the schematic text is too small to read. Can you post a bigger pic?

    1. That chip is indeed shiny. Sadly, the way I read the datasheet is that it has undervoltage protection on charge, trickle charging a deeply-discharged battery until it is safe to fast charge. That’s all well and good, but I’m trying to work out how to stop the battery getting into that state in the first place, i.e. undervoltage protection on discharge.

      It does say that “The USB controller can set the ISEL pin low to charge the battery at 500 mA.” So I still need a USB controller? Or can some random microcontroller not bother talking to the USB host, and just request that the battery charger handles 500 mA, whether that’s available or not? Coz I’m thinking that having to implement and squeeze in a USB controller is going to be a pain in the arse just to be able to ask the USB host nicely for that fat 500 mA.

      Oh, and LLP package? Argh! But I shall have a look at National Semi’s other choices later, after I’ve drunk beer, which always makes reading datasheets more fun.

  1. Oh yeah, LLP. Fuck trying to solder those by hand.

    If you don’t have a controller, just pull it to 500mA and it’ll be fine. Well, for values of fine that include drawing 500mA from something which can’t supply it but we’ve had that discussion.

    Strictly speaking, if you *did* have a USB chip there, you’d tie it to the controller’s power good signal, and program the chip to request 500mA.

    I’ve also seen similar chips with 2-3 LDO regulators built in as well, with per-rail power control. Maybe those include undervoltage detection.

    (I admit, I’m much more of a monolithic chip guy than a discrete logic/analog)

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