Lithium Ion Battery and Warm Circuit Board

I’m curious if I have a legitimate concern or if I’m just being overly paranoid.

I have a rechargeable Lithium-Polymer battery (prismatic shaped) rated for a max charging temperature of 45 °C.

The battery rests against a circuit board that I know can generate some heat when charging the battery (500 mA current via an MCP73831 IC). It can get up to around 49 °C in a single location on the board (say about 1/4”x1/4” in size) and a dissipated temperature around that (I even measured 63 °C once on the hot spot but haven’t been able to reproduce it). That is for the back of the board where the battery rests (the front side can get up to 80 °C at the charger IC).

Is there any legitimate concern that this will heat the battery during charging? There is nothing between the LiPo and the board. Using a thermal camera, it doesn’t seem like any heat really transfers well to the battery, so it seems like it should be okay, but I’m no expert in heat transfer, especially if it remained in the described state/position for a few hours. The battery and board would sit inside a plastic enclosure.

• Can you give these temperatures in Celsius? All Datasheets I've ever seen use these units to describe temperature ranges, and especially thermal resistance. Also, they are international standards. Believe it or not, I'd need to go to wikipedia to even get a remote feeling for what 175°F is. – Marcus Müller Jun 1 '19 at 7:27
• Done. Please take a look. – menehune23 Jun 1 '19 at 12:46
• ah, cool. upvote! – Marcus Müller Jun 1 '19 at 13:42

Lithium-Polymer service life is seriously degraded at high temperatures, especially when fully charged. The cooler you can keep the battery the better. I suggest using a more efficient switch-mode charging IC such as the TP5000.

Now, the question is: How much of it do the powerful chips that generate a lot of heat ("heat" is thermal energy) transfer into the battery; that's a question of considering where the heat goes.

First thing to realize is that if you have a perfectly sealed, perfectly thermally isolated box, it's going to heat up forever, and at one point break down/melt/outshine the sun in its temperature.

Obviously, that's not happening, because the enclosure of your device isn't a perfect thermal isolator.

Conversely, your board itself isn't a perfect thermal conductor, either: if that were the case, all spots on the board would instantly have the same temperature!

So, in a first step, it'd be important to model how warm your overall device gets. Lets consider it as a black box: Inside, someone converts $$\P\$$ watts of electrical power to heat, and these will need to be dissipated to the environment in order to stop the infinite heating up.

Now, the way we model that actually uses similar terminology as we're used from Ohm's law: There's thermal resistance, that tells us how much something is in the way of heat flow. Its unit is typically "K/W", or "°C/W" and tells us how much hotter something gets if a specific power is converted to heat inside.

You'll often find IC datasheet specifying something like a "junction to environment thermal resistance 45 °C/W", and together with an estimate of how much power the IC uses (for example, voltage drop times current in a linear voltage regulator), you can tell how much hotter than ambient things get.

So, our process goes like the following:

1. Estimate how much power is converted to heat in your system.
2. Estimate the thermal resistance of your enclosure; that times the power from 1. gives you how much hotter the inside of the enclosure is than the outside

From here, I'd guess that in any typical device, improvements are minor by being more detailed: If you're already above 45 °C, then you're not colder anywhere inside the box (after a while, at least), and your device needs better cooling.

If you're sufficiently below, and there are enough places heat can go without going through the battery, you honestly don't need to worry too much.

Problematic would be if you're close below 45 °C inner-enclosure temperature; then you'd need to calculate further:

1. Estimate how much warmer the components in close proximity to the battery are than the in-box environment: same procedure as above, but ambient temperature is the already elevated one of the inside of the box.
2. calculate the heat transport that reaches the battery by putting all thermal resistances in parallel between the heat source and the battery and calculate how much heat will flow into the battery.

Step 3. and 4. are pretty often done in simulation, because estimating how much heat a complex PCB and a battery fixation will transport is hard.

Step 1. and 2. can be done pretty well by hand: For the outside of the box, you can often assume something like "well enough ventilated place" and hence assume cooling by convection and maybe radiation. There's ready-to-use formulas that relate horizontal and vertical surface area to the resulting thermal conductivity and resistance.

• Thanks for the detailed answer. It’s really only a small spot on the bottom of the circuit board that get above 45 deg C. The average temperature of the board is likely well below that (say 30-40 deg C). I’m just paranoid about whether that warm spot could heat the whole battery, or whether I’m just paranoid. I see many similar designs using a TP4056 LiPo Charger IC at a 1A charging current and have never heard of a battery exploding on somebody. – menehune23 Jun 1 '19 at 21:39