Using 21700 batteries in 18650 solar product

Hi guys,

Hoping to get your advice :slight_smile:

I am wanting to use the Solar Power Manager (C), Supports 3x 18650 Batteries, Multi Protection Circuits from Waveshare.

The only problem is it allows for only 3x 18650 batteries @ 3.7V, where I was hoping to use 3x 21700 @ 3.7V for the additional run time.

Here is a photo of where the battery cells go:

I spoke to Waveshare and they advised we should be able to use 21700 batteries (plus I did some research and they are apparently the same as long as they have the same voltage). The only issue is I do not believe it will fit into the enclosure due to the battery dimension being larger.

What is the best way I can integrate 21700 batteries to use with this device?

My thoughts:

From my understanding the batteries are being used in parallel for the purpose of battery capacity. Could I use a 3 slot battery holder such as this, and then solder the red/black wire onto the battery springs?

Screenshot 2023-09-06 135438

I am not exactly sure how the black and red wire would be wired to the battery springs in the Solar Power Manager though.

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Hey Nick,

Welcome to the forums!

I cant see any reason for 21700s to be incompatible here other than the size difference. Though wiring will be incredibly important so that you don’t short them out.

Have you got a link to the product you are using? We can have a look at it and try to see how the batteries are wired in that one to help as much as possible.



Hi Nick
If you look closely it would appear the lower photo seems to have the 3 batteries in SERIES to give a total of 11.1V. If they were in parallel the springs would all be on the same end as in the upper photo.

So NO you can’t use this box.
Cheers Bob


Hi Nick,

I had trouble finding that part on the Core website, do you have a link?
Waveshare are pretty good (most of the time) when it comes to schematics.

Multi-cell LiPo packs should have balance charging so will need at least 4 leads.

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Hi Blayden,

Here is the product link: Solar Power Manager (C), Supports 3x 18650 Batteries, Multi Protection Circuits

Product manual: Solar Power Manager (C) - Waveshare Wiki

Would be great to get insight on how the batteries are wired as I am not savvy when it comes to this stuff.


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Thanks Rob, I was wondering how to differentiate series from parallel - legend!

I will seek for an alternative box in the mean time.

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Hi Liam,

I just linked the product below in my response to Blayden, unfortunately Core Electronics does not sell this product.

I could not find a schematic on the product page, however if it is absolutely required I can of course email Waveshare directly for this - fingers crossed they have one they can send.

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Hi Nick,

Thanks for sending through that link!

Since Core doesnt have any for testing and the schematic isnt available you’ll need to get one to test or email Waveshare for a schematic.
(Its a shame it isnt available as a bare PCB with a terminal block or something for batteries)

Core’s product manager is on the forum so might be able to get it online for you :slight_smile:

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Hi Nick

Parallel means what is says, side by side positive to positive neg to neg and so on. The end voltage is the same as individual cells.

Series means end to end, positive to negative (NOT the same battery) positive to neg and so on until you run out of cells. The end voltage is the sum of the cell voltages.

If you look closely at the lower photo you can see how the end contacts are connected together. Series configuration.

Methinks it would not hurt to do a bit of research and reading before you do yourself an injury. And yes, these cells can explode or catch fire if mis-used.
Cheers Bob

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Hi Robert,

Makes perfect sense thank you robert!

I believe I have found a suitable product (comes with two separate items one for parallel and one for series: 2Pcs 21700 Battery Storage Case 3 Slots x 3.7V for 3×21700 Batteries Holder Box Container with ON/Off Leads and Switch PC Plastic Material Nickel-Plated Shrapnel Transparent case – Bistook

I guess I could also manually connect them in parallel using something like this:

Thanks Liam,

I have just emailed Waveshare for a schematic, they did just get back to me though on my original query:

If you need to connect your own battery box, please set the positive terminal of the battery to red and the negative terminal to black.

Not sure if that gives any insight on the wiring - awaiting for the schematic at this point.

P.s. I just linked a couple of battery boxes I believe may be suitable below to Robert :slight_smile:

Hi Nick
Yes, the parallel one of the first link or the second one will do as you can connect whichever way you like and it looks like the individual holders will lock together either way around so one could make up a custom battery fairly easily.
Cheers Bob