Moving from SLA to LiFePo4 - charging

Thread Starter

Bob21

Joined Oct 19, 2021
9
Hello,

I am hoping this is the right forum to post this on, although I’m afraid I’m probably woefully under qualified to be here! :D

I have an old block rocker portable speaker made by Ion:

A4204D5C-7EAA-4EDA-8654-A9CE29C5C4B1.jpeg
It’s a very old model, specifically IPA06. When I bought it over 10 years ago, it was pretty good solely because it was loud and ran for 12 hours, something which many smaller units do nowadays anyway but with a more compact higher fidelity package

I wanted to give it some new life and make it useful again so I have modified the drivers and removed unnecessary features such as the 30 pin Apple iPod dock at the top. This process has required gutting the existing circuitry and putting new class D amplifiers.

It used to run on a 6V SLA battery of 7Ah capacity, and contained a 240V transformer with internal charge circuitry. The new circuits run on 12V.

I have been looking at 12V LiFePo4 packs of approx 10Ah, such as this:

https://www.batterymasters.co.uk/li...x-charge-discharge-current-weight-1-2-kg.html

The battery should fit nicely in a new location within the housing and has a high enough rating for the new amplifiers. I have added a DC jack on the rear of the cabinet and have a 12V 6A PSU in an enclosure (typical laptop style brick) so the 240V step down will not be internal anymore.

However: I am not sure how to go about charging a LiFePo4 battery or how to wire the battery so that it’s disconnected from the circuit when charging. I have found conflicting information about charging LiFePo4s, some that says you can charge them with Lead Acid chargers and some that says absolutely not. The logical head on me says not!

Am I right in thinking that to a certain extent, because these batteries have BMSs in them, a lot of the ‘hard’ work is already done- I just need to find some charging circuitry that will apply 12V to the battery and cut off at a certain point?

I was looking at something like this:

https://a.aliexpress.com/_u9aoli

Or this:

https://a.aliexpress.com/_vYqmlK

although I am sure that these are probably a bad idea for several reasons, so I am open to any suggestions to how I can get this circuit running on battery power again.

I can competently solder and put a circuit together, but I’m just not sure what it is that I actually need. Any and all help would be thoroughly appreciated :)
 

Audioguru again

Joined Oct 21, 2019
3,833
The ad for the expensive battery from England says, " Complete with Fast Lithium battery Charger" and also says, " Also supplied with its own lithium charger that fully charges the battery in around 2 hours from flat".
So you do not need to add any cheap Chinese junk to it.

Isn't the speaker a single mono unit? What is its impedance?
With a 12.6V battery, a class-D bridged amplifier (TPA3116) produces about 7W into 8 ohms or 13W into 4 ohms per channel (it is a stereo amp with 2 amplifiers) at low distortion.
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
Hi :) thank you for reading and replying

Yes there is a charger supplied with it. It’s this: https://www.batterymasters.co.uk/ul...ifepo4-battery-pack-110-240vac-ce-listed.html

It would require disconnecting the battery like having it on a bench. I’m talking about charging the battery in place from a 12V supply without interrupting operation.

Yes the original speaker arrangement was mono. It had a single larger ~8 inch midwoofer and single tweeter. I got given a couple of small 2-way car speakers and a 2.1 amp from an old caravan refurb. I have fitted these with the woofer and removed the tweeter.

I have looked for the chipset but I cannot find it, I suspect it might be a TDA range (it’s this: https://m.fasttech.com/product/9637221-hongxing-hx-168-2-1-channel-super-bass-hifi) The PSU it came with is 12V 5A and even at half volume it’s more than enough.

The woofer and 2x 2-way car speakers are all 4ohms.

I will measure the current draw later after work but I don’t think we’re looking at more than 3-4A.

You said that battery was expensive, where else would you buy that is not so expensive?
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
I have measured the current draw with the volume as high as it can go without distorting and we barely hit 2A. This leaves lots of wiggle room on the 10A lithium I was looking at. Very tempted to add a USB charge port and possibly a 2nd amp and additional drivers on the reverse.
 

Audioguru again

Joined Oct 21, 2019
3,833
A meter cannot accurately measure the very short duration of audio peak currents from a battery. Even the average current measurement will be low.

The battery can always be connected to the amplifier when charging overnight with the amplifier turned off. The charger will power the amplifier during daytime but with not much charging of the battery.
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
A meter cannot accurately measure the very short duration of audio peak currents from a battery. Even the average current measurement will be low.
I realise this. I figured it’s probably double the reading for peak. That’s still only 4A and well within the linked battery. Probably need a better PSU though.

The battery can always be connected to the amplifier when charging overnight with the amplifier turned off. The charger will power the amplifier during daytime but with not much charging of the battery.
This isn’t the desired result. I have a PSU, or can get another if it’s not powerful enough. I need help building a circuit that will power the amp from the PSU (and disconnect the battery from the circuit whilst it’s charging) and switch back to the battery when unplugged. Kind of like every other device…. No one at a party/function is going to want to / remember to hook up a battery or not turn up an amp so much or not have music whilst it charges for however many hours.
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
There is a small possibility that I could obtain a charger that could be opened and the mains transformer removed, and just feed it directly with 12V, but then I would still need some circuitry to disconnect the battery from the amp while charging.
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
Use a switched connector, they are common on barrel jacks.
Hi :) good idea.

I was hoping I could find a LiFePo4 charger that runs on 12/14V that I would be able to use with something like that but I am having difficulty.

If it’s not possible to either find a 12V source charger or successfully gut a 240V one, what other options are there?

Is something like this not suitable?

https://a.aliexpress.com/_vrowum

Fed into something like this:

https://a.aliexpress.com/_vZ7Hz4

The BMS would disconnect to prevent overcharging anyway?

I know a lot of these boards are cheap Chinese junk but I’ve been looking through a lot of the ‘intelligent’ chargers for LiFePo4 batteries and they all look like cheap Chinese junk… below about £120.. and even then the ones above that only look better because they’re in an aluminium enclosure. I doubt the PCB is much better… some of the most popular units on the usual sites have reviews that tell of smoking melted chargers.
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
Battery is here :) So assuming no charging circuitry and the battery is removed to charge with a dedicated lithium charger, this can just been hooked straight to the amp with an on/off switch on the positive wire? No other special considerations needed?
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
The battery has a built-in low voltage detect and disconnect circuit that might surprise you when it functions. Maybe you need a battery voltage meter.
Yes, I will get one. Does the protection circuitry also deal with over current? Obviously fuses will be added also.
 

Thread Starter

Bob21

Joined Oct 19, 2021
9
Maybe. I’d rather not take the risk of a lithium fire though so looking at the battery, it’s label says:

07E10598-AF04-41D8-B04B-05E9FEADF3CC.jpeg
So, if I’m not mistaken, that means the battery can take a peak discharge current of 20A. So constant is probably half that? Each amp should consume no more than 4A (and a 4A quick blow fuse per amp will be fitted). There’s also a Bluetooth board I will add to feed the amps and this is rated at 1A and a 1A fuse will be added. So if it’s as simple as adding the numbers, that would equate to 9A.

This seems sound to me but I’m skeptical because of the lithium factor. It’s a simple circuit that literally goes battery > fuses > amps, nothing else.

Is this sufficient or is there something else I need to add/do?
 
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