Cheapo Battery Management System

[jkeny]

I'm about to use 3 of these LiFePO4 batteries in a USB DAC. These are 3.3V and I will be using 3 of them - 1 to provide 3.3V directly to digital chips on a DAC board & another 2 (with a series R voltage knock-down) to provide 5V to the relevant chips.

Some issues:
- I thought of using some of the many mobile phone 3.3V chargers that have gathered in the house over the years. It is recommended to charge batteries individually even the ones in a battery bank so as to balance the charge across all the batteries evenly. I thought I would have the 3 charges inside my DAC box & just plug it into the mains when not in use. One question, these mobile chargers automatically stop charging when they have reached a threshold - is this cut-off circuitry typically in the charger or in the phone?

- I wanted to avoid a regulator to bring the 6.6V 2*battery pack down to 5V because I wanted to compare the raw battery source against various battery/regulator sources. An R will burn off some excess voltage for me, I believe. How I calculate the needed R is by knowing the current draw on the existing 5V. Is the best way to do this to put a 1R 5W R in the existing 5V PS line & measure the V drop across it? Then I can calculate the needed R for my proposed 1.6V drop.

- The final thing I hope to do is have some LEDs on the box to indicate when voltage drops below 3.3V or 5V respectively - any quick schematic for this?

[Bakmeel]

Hi Jkeny,

- The battery charge management for cell-phone batteries is normally inside the phone itself. It is then often software-based, or a dedicated part of the power management IC's. The adapters that come with your phone are usually very simple DC power supplies with no battery management.

- If you have a multimeter with current measurement, then this would be the simplest option. Otherwise, a 1R / 5W resistor would also do the job. I suppose you're note expecting many amps to flow right?

- Google around for "under-voltage detector" or "battery low-voltage alarm" or such... I'm sure you'll find something.

[jkeny]

Originally Posted by Bakmeel

Hi Jkeny,

- The battery charge management for cell-phone batteries is normally inside the phone itself. It is then often software-based, or a dedicated part of the power management IC's. The adapters that come with your phone are usually very simple DC power supplies with no battery management.

I suspected as much - that's a pity!

- If you have a multimeter with current measurement, then this would be the simplest option. Otherwise, a 1R / 5W resistor would also do the job. I suppose you're note expecting many amps to flow right?

not many amps flowing in this DAC!

- Google around for "under-voltage detector" or "battery low-voltage alarm" or such... I'm sure you'll find something.

Will do, thanks

[woodturner-fran]

John,

theres a sch on the DDDac site for just such a LED system. He is using 12V battery and he has it set up that if V>11.5 a green LED lights and <11.5 a red one comes on. It can also be set up for 2 colour LEDs.

Might not be that big a stretch to have it hooked up to a relay to allow charging automatically?

Fran

[jkeny]

Thanks Fran,
I'll have looksee, sounds like just what I need!

[BZed]

You should have a lok at this before you get too far, just so you understand the charge/discharge needs of your batteries.

http://www.batteryuniversity.com/

Once you know this stuff you understand why the micro controlled charging circuits are around.

BZ

[Saturnus]

Originally Posted by jkeny

- I wanted to avoid a regulator to bring the 6.6V 2*battery pack down to 5V because I wanted to compare the raw battery source against various battery/regulator sources. An R will burn off some excess voltage for me, I believe. How I calculate the needed R is by knowing the current draw on the existing 5V. Is the best way to do this to put a 1R 5W R in the existing 5V PS line & measure the V drop across it? Then I can calculate the needed R for my proposed 1.6V drop.

Easiest way I can think of is to have a diode over both the plus and the minus giving a 1.4V drop in total. But you might want to look into a real ultra low loss regulator as the voltage from the LFP cells will vary from about 3.6V fully charged to about 3.2V when almost discharged.

[jkeny]

Originally Posted by Saturnus

Easiest way I can think of is to have a diode over both the plus and the minus giving a 1.4V drop in total. But you might want to look into a real ultra low loss regulator as the voltage from the LFP cells will vary from about 3.6V fully charged to about 3.2V when almost discharged.

Thanks Saturnus, you seem to have some knowledge of these battery types - have you used them in audio?

I guess as long as the voltage stays within the PS compliance range for the chips it may be OK but I intended to keep the batteries topped up to full charge (will prolong their life, as long as they're not overcharged). My chips are the Cypress EZUSB-FX2 (3.15 to 3.45V);
Xilinx Spartan 3A VCC(3.00 to 3.6V), VCCint (1.14 to 1.26);
PCM1793 VDD (3.0 to 3.6V), VCC (4.0 to 5.5V)

I was hoping to stay away from any regulators as I believe & have seen reported, that any & all regulators imposes their signature. This might be mistaken but I wanted to evaluate it's sonic effect

[Saturnus]

Originally Posted by jkeny

Thanks Saturnus, you seem to have some knowledge of these battery types - have you used them in audio?

Yes, I have been testing them for use in my next boominator. And they are a no brainer for that. Best battery on the market. Period.

[jkeny]

Originally Posted by Saturnus

Yes, I have been testing them for use in my next boominator. And they are a no brainer for that. Best battery on the market. Period.

I'd be interested in your impressions of these batteries powering the Tripath amp as this is my next step - I have a TA2020 amp that my transformer output DAC is DC connected to - sounds excellent with SMPS 12V power - will be trying a Salas shunt reg & then the batteries.