How to add a Li-Ion charging circuit to board design

[auriuman78]

Hello world!

I've currently got a tube headphone amplifier that I built using a 12au7 tube. It's powered by a 12v SLA battery / linear external 12.6v dc supply with a rectifier valve to check the battery when on external. The SLA battery is not performing to what I'd hoped however. I was going to upgrade it to a Li-Ion 11.1v battery which would work better with the rectifier valve anyway due to lower voltage. My current dilemma is how to incorporate the charging circuit into my board design so I don't have to disassemble it to charge the battery every time it dies. SLA is easy. Li-Ion is a bit more sensitive from what I've seen since they are constant voltage with controlled sensed current. I work with those circuits daily but I don't know much about designing one. I've tried searching but most of the data I've found on charging circuit design does not include how to add such a circuit to an existing design.

I would love tips, experience and links to design documentation on this topic. I do want to understand the technical details behind the design and not just grab any old circuit design and integrate it into my amp. Thanks in advance!

 

[Tommy_Ent]

Here's the voltage limiter I'm using with my Li-Ion 11.1v battery pack. All it does is charge up the battery and switch off the input power when 12v is reached. All you need to do is build this in parallel to the rest of you circuitry and plug it in when it needs a charge.

 

[auriuman78]

Thank you very much. The help is appreciated. I'll post after I've implemented it to report my results!

 

[davenn]

Tommy,

how did you do the switching ?

the circuit you have shown is just a constant current supply as per the data sheet, no switching included. Have you the rest of your circuit that shows how you did the switching.
That may be of great help to auriuman

Dave

 

[(*steve*)]

That circuit is a voltage regulator with a high output impedance to limit the maximum current.

It's not bad for charging LiPo batteries, but certainly not optimal. Correctly set up it may be a little slow, but it will be safe.

It's not really appropriate for other battery technologies, although -- ignoring variation of voltage with temperature -- it's borderline acceptable for lead acid too.

 

[auriuman78]

I had planned on keeping my original rectifier check circuit in place to prevent back current. Would that not work in this setup? Previously I'd been using a 12.6v 1.2A DC pack direct to a 12v SLA battery but I had to keep track of how long I'd been charging it. I know Li-Po and Li-Ion are a bit more sensitive than that, you can't just hook em on up to a supply current.

 

[KrisBlueNZ]

You can get ICs specifically designed to control the charging of Li-ion batteries (and other chemistries). These will give best performance and safety. A quick search on Mouser shows that battery charge controllers are made by Microchip (www.microchip.com), Texas Instruments (www.ti.com), Maxim (www.maxim-ic.com), STMicroelectronics (www.st.com), and a few by Vishay Siliconix and NJR.

Unfortunately, Microchip's portfolio only includes single-cell and two-cell chargers, and Texas Instruments and STMicroelectronics only have single-cell chargers (for Li-ion cells at least).

That leaves Maxim, who have many options that support three-cell batteries - see http://para.maximintegrated.com/en/results.mvp?fam=batt_chrg&168=Li-Ion

All of these devices are in SMT packages. You would need some kind of adapter to use them with stripboard or similar protoyping or hobbyist construction systems. Google "SMT to THT adapter" - there are many solutions out there.

These components may not be available in one-off quantities. Certainly, Maxim will only sell you a reel of them, unless you can convince them you're designing a product for mass production, in which case they might send you some samples. Check Digikey and Mouser to see which ones are available singly.

 

[auriuman78]

Most excellent advice, thanks for that. I'll be checking those out shortly - I'll report back what I find and my results with said choice.

 

[auriuman78]

Ended up choosing the MAX1873 at Maxim. It's a QSOP smt 16 pin package so I'll be able to hand solder it to my pcb. Just ordered it. I'll be sure to let you know how it goes once I get it. It's pretty cost effective too, $1.98 ea and they do sell them singly. I did purchase 4 just in case I blow one up connecting it. Thanks for all the help fellas!