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Imperf3kt
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Using batteries with multiple boost converters

Tue Jul 24, 2018 4:53 am

I've got a project that utilizes a Pi0w and a 12v accessory. It has to be portable, so I'm using a lithium ion 18650 and a boost / charge module.

On its own, this setup works perfect, my issue arises when I want to add a 12v accessory. I want to avoid boosting the boosted 5v to 12v because I aim to draw more current than a single boost unit can handle. My idea was to add a second boost module to the battery, which has a variable input and output. I believe this should work as long as I stay below the C rating of the battery, but I am unsure what happens if I try to charge the battery. Is it okay to do this or will I see a higher voltage than 12v coming from the second boost module?

Here's a rough diagram for context.
dual boost.png
dual boost.png (11.92 KiB) Viewed 722 times
Note that I accidentally mislabeled the output of the second boost module as 5v, it should say 12v.
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Z80 Refugee
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Re: Using batteries with multiple boost converters

Tue Jul 24, 2018 7:50 am

Seems like a reasonable plan to me.
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mfa298
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Re: Using batteries with multiple boost converters

Tue Jul 24, 2018 11:39 am

Imperf3kt wrote:
Tue Jul 24, 2018 4:53 am
My idea was to add a second boost module to the battery, which has a variable input and output. I believe this should work as long as I stay below the C rating of the battery
That should be ok. Obviously ensure the 2nd boost can handle the current you need at 12v as well. And for the discharge rating of the battery ensure you take into account both the 5V load from the Pi and the 12V load from the other stuff. Also don't forget the battery voltage will drop as it discharges so the current draw will potentially increase.

Imperf3kt wrote:
Tue Jul 24, 2018 4:53 am
but I am unsure what happens if I try to charge the battery. Is it okay to do this or will I see a higher voltage than 12v coming from the second boost module?
That's liable to depend on the quality of the boost modules (a low quality 5V one might also vary it's voltage as the input changes). As above the battery voltage will change as it discharges so the voltage of a fully charged battery might be similar to the charge voltage. When looking at the charger setup you might also need to take into account whether it's capable of running the hardware as well as charging the battery. It might be worth looking at dedicated PMIC chips that can handle all parts in a single device.

mosespi
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Re: Using batteries with multiple boost converters

Wed Jul 25, 2018 12:09 am

As long as your boost converter and battery are all within their specifications I would not anticipate any problems when NOT charging.
It's another matter when the battery is charging. Charging lithium batteries is a bit of a delicate process, and introducing a load to the battery that the charger was not designed for might result in unexpected behavior. The battery may or may not fully charge, it may be in a state where it is constantly charging and discharging due to the now confused charge circuit. You may be unnecessarily cycling your battery because of this and it may get hot. You might have a load that is potentially varying.. there are a whole host of problems here, some of which also effect USB battery banks that are used while charging (ala UPS).

If you can, avoid drawing from the battery (charge circuit) while it is charging, because the manufacturer of the MP2636 converter didn't design for this, and we don't know really what is going to happen. Best case you chew up your battery a bit quicker. If you can cut over your 12v boost converter to your input power, either manually or automatically, while charging you will avoid this problem.

One more item of note, you may already know this.. if your boost converter bypasses any low voltage discharge protection/disconnect/BMS you should provide some other form of protection. Damage results to lithium batteries taken below a certain voltage.

Regards,
-Moses
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Imperf3kt
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Re: Using batteries with multiple boost converters

Wed Jul 25, 2018 1:44 am

Yeah, those are pretty much my exact concerns, Moses.
I've been doing a lot of testing and experimenting with the MP2636 over the last few months (although, sporadically) with a Pi3b (I think you commented in a thread I opened about that, too) so I have a very good idea of how it behaves when connected to a battery by itself.

The unit I am using has been designed to allow parasytic loads, and I've confirmed the unit pretty much fine when used as such. The charge current is set too high for use with a Pi3b however, and attempts to adjust that failed, but a Pi0w doesn't exhibit the same symptoms.
My concern here, lies in behaviour of the secondary boost module which is purely a step-up converter only.

What if I got a constant current, constant voltage module? These are more expensive and bulkier, but shouldn't care about any input voltage fluctuations, right? (obviously as long as it stays within spec)
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Re: Using batteries with multiple boost converters

Wed Jul 25, 2018 7:09 am

1. I assumed you would not have any 12V load while charging. If you want to take load from the battery via module 2 while module 1 is charging the battery, you could be in a world of hurt.

2. What do you mean "constant current, constant voltage"? Which module do you mean, performing which function? If you are talking about the charging function, constant current and constant voltage describe different phases of the charging cycle and the management system shou!d be using them in a programmed manner according to the battery type and state of charge. In particular, however, constant voltage is used for lead-acid batteries, and constant current for everything else.
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Imperf3kt
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Re: Using batteries with multiple boost converters

Wed Jul 25, 2018 8:45 am

Oh really?
I should probably go research those a bit more then.

I can live without the 12v stuff while charging the battery. This sounds like the safest option.

Hmm... Space permitting, I may be better off putting a buck down converter in to do the charging of the battery and supply 12v where needed directly.

Maybe, I don't know. This probably warrants more thought, thankyou for the ideas and information.
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