I want to power my Arduino project with lipo batteries. I want to hook up 2 batteries and use a solar charger to charge one battery while the other is powering my Arduino. Looking for a circuit that would do this or has anyone done this before. Switching from a low battery to a newly charged battery without any power loss. Thank you all for the help.
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What a strange requirement, unless there’s some not yet explained reason for doing this.
Normally, you design your charge & storage to run the device for as long as needed.
It’s rare to plan for less than the required capacity and change batteries when they run low.
That idea went out with primary cells in the 80s.
Sorry did know it was such an outdated question. Thank you for your feed back.
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I don’t want to shut you down, but there are hundreds of suitable solar / Li charger / battery controllers on eBay and other sites.
For what you need it sounds like the answer will cost less than a tenner.
If you prefer older style battery chemistries, there are really simple methods for maintaining NiMh cells.
Hi Sprinkfitter,
Welcome back. Hope you're able to complete this mission into enemy territory without taking too much flack from an enemy intent on defending their territory 
I've worked with LiPos a lot with RC helicopters ... they're interesting beasts.
If you want to switch from charged one to a discharged one (or vice-versa) without any power loss then possibly one way to do it would be to momentarily parallel them so that the freshly charged one is momentarily also connected into the circuit before the discharged one is disconnected. In THEORY people often think that would create an extremely high current situation but in PRACTICE it's usually not a problem because they have internal resistance and the difference in voltage potential is usually quite small anyway. In a way this technique is a bit like jump-starting a car.
So I'm wondering if you really just need a couple of single poll double throw relays to connect each battery between either the Arduino or the Solar charger - controlled by the Arduino (or a separate one if you wanted). Just sense the voltage of each and take action as required.
You need to be careful charging LiPos - they bite back hard if over-charged (swelling -> fire) - and go "real bad, real fast" if you over-discharge them. And temperatures above mid 40 - 50 deg C start to decrease their lifespan too.
Having just said all that, I'm wondering why you wouldn't just leave the battery connected to both the Arduino and the charger?; it shouldn't be a problem.
Hope this helps.
@CJSouthern Sorry, was that for me ?
I might’ve missed part of the conversation.
If you have two batteries, it might make more sense to simply use both of them in parallel, and let the solar panel charge them together when the sun is out. You might well get just as long battery life, particularly if the alternative involves driving relays.
But in general, the charger modules I've seen on Ebay, such as the ones using the CN3065 or TP4056, are designed to power the load in parallel with the battery, so load current provided by the solar panel goes through the charger. For some projects that may work ok, but for others it can present real problems. It depends on whether the project load current exceeds 10% of the full current charge rate the charger is set for. If it does, then the charger may not start up properly, or terminate charging properly, and in any case when the battery is fully charged, the charger IC will terminate charging, which means the battery has to power the load even if it's high noon on a clear day.
The solution is called a "load sharing" or "power path" circuit, which lets the solar panel directly power the load when the sun is out. The typical load sharing circuit works for USB power, which is a fixed 5V, but it doesn't work properly with variable solar power. I'm going to be testing what I hope is a better circuit as shown below. I'm not aware of any Ebay solar charging modules featuring load sharing circuits other than one using the MCP73871 which I've never tried.
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you have to learn a lot of terminology when moving to LiPo
most common battery: 18650. 18 mm diameter, 65 mm long. there are smaller round batteries, and flat packs
BMS: Battery Management Systems. if you have three batteries, you need three separate chargers, wired in series. the negative of the charger in the middle is the positive of the bottom charger; the positive of the charger in the middle is the negative of the charger on top.
this is a 3S 1P ( 3 series 1 parallel BMC charger for 18650s:
3 X 3.7 = 10.1 VDC, which they call 12 VDC. here is a smaller one without holders:
https://gdiye.co.za/online/index.php?route=product/product&product_id=613
you need an MPPT controller to cut solar panel power down to BMS system level:
there are hundreds of tutorials and youtube videos explaining how to combine these.
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