Battery Management System for a cellular Data Logger

Thread Starter

Dan Simmons

Joined Jan 11, 2019
12
Hi,

There seem to be an enormous amount of Battery Management Products out there and I'm having trouble choosing one to go for!

In a nutshell, we are developing a remote data logger which uses a small LiPo battery for power, along with a larger Li-SOCI2 (2 in series gives 4.2v-7.2v) and/or solar panel battery for recharging.

Our cellular based system uses 5v, is usually in deep sleep mode (<1mA), and wakes up every so often at around 80mA, but can have 500mA surges when transmitting data.

We'd like the LiPo to be the primary source of power, and have the Li-SOCI2 or solar panel as interchangeable/combinable methods of recharging the LiPo battery.

Can you advise which product you would go for please? Don't mind which brand; TI, Microchip, Analog Devices, ST etc.

Many thanks for any advice and experience you can share!
 

Thread Starter

Dan Simmons

Joined Jan 11, 2019
12
So I got in touch with a Microchip system advisor, and he said to look at the AN1149 note, along with the MCP73871 datasheet.

However, it shows a typical either/or system where the load is powered by the power supply (if available), and the battery if unavailable. For our application, the power supply is ALWAYS connected, but we only want the BATTERY to supply the load!

Am I missing something here?
 

nsaspook

Joined Aug 27, 2009
6,270
What's wrong with the either/or design in you case? Is there a specific reason for the system load to be supplied primarily by the small LiPo battery?

From AN1149
The input power should supply the system load and charge the battery when a battery is present in the system. When the input power source is removed, the system is supported by the battery. When the system load and the battery draw more energy than the supply can offer, the system load takes priority over the battery charger.
 

Thread Starter

Dan Simmons

Joined Jan 11, 2019
12
Sorry, I should have stated…
The power supply in our case will be several Li-SOCI2 batteries in parallel.
The method is to use the smaller 2000mAh LiPo battery by default to handle the load with its spikes when powering up and connecting over cellular.
We really need the Li-SOCI2 supply to trickle charge the LiPo as Li-SOCI2 batteries last much longer without spikes.
This chart shows Li-SOCI2 capacity vs current:

image

As you can see, in ambient, 20mA would be optimum.
In fact, ours will be outside so the optimum would most likely be somewhere between 5 and 20mA.
 

nsaspook

Joined Aug 27, 2009
6,270
So you are using primary (non-rechargeable) Li-SOCI2 batteries to trickle charge a LiPo battery. Everything I've seen says Li-po must never be trickle-charged.

A BMS can do a proper recharge to match operating conditions but it won't be a trickle-charge.
 

Thread Starter

Dan Simmons

Joined Jan 11, 2019
12
OK that's good to know! Would 20mA be classed as trickle charge?
Even so, is there a way to at least limit the current drawn from primary power supply so that peaks (say, larger than 50mA) are handled by the battery?
 

nsaspook

Joined Aug 27, 2009
6,270
Last edited:

Thread Starter

Dan Simmons

Joined Jan 11, 2019
12
This seems like a good solution! However the voltage would actually be between 4.8v and 6.4v depending on battery charge. What happens if the input voltage goes above the 6v absolute maximum in this circuit? I could not find out.

After doing some experimentation, I have found that I can simply use a 100 Ohm resistor in series with the supply voltage to limit current to around 24mA (in active mode). I'm sure this will have downsides, but because the power dissipated is negligible and the current doesn't need to be strictly limited to a particular value, what are the benefits of using the MIC2090 compared with a resistor in this application?
 
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