Car amplifier with 4x lithium in series

Thread Starter

DMahalko

Joined Oct 5, 2008
189
(I am putting this thread in here as this involves car audio amplifiers and car voltages, but the amplifier is not going to be used in a car but just used standalone.)


I want to power a small 100 watt RMS 12v car amplifier using lithium ion batteries, to wirelessly drive a bass shaker on a computer chair with casters, so that I am not constantly running over the power wires to the shaker.

Is there a product available that already does this without me needing to hack something together myself? Personally I'd like to have something sophisticated with a battery temp monitor for charging each cell, so the batteries don't blow up or melt while charging, or if the sun is shining on the battery packs, etc.


It's difficult to talk about this with precision, since lead acid batteries are nominally 13.6v charged, and a vehicle charging system voltage can be 15-16v or so.

Meanwhile lithium batteries average about 3.6 to 3.7 volts fully charged, so four of them in series is 14.4 to 14.8 volts. As such, the full charge voltage is a bit high vs lead-acid, but this is nominally in a compatible range with what an amplifier would see, installed in a car with the charging system active.


The problem is charging the packs in series without disconnecting them from the circuit, and without the amplifier seeing the total charge voltage. Apparently the fully charged voltage is 4.2v per cell which for four is 16.4 volts. This sounds like it is dangerously close to the voltage limit for a car amplifier.

I'd like charging to be as simple as "plug it in and it just does it", but would it be better to install an On/On DPDT switch to fully isolate the charging circuit from powering the car amplifier?
 

Thread Starter

DMahalko

Joined Oct 5, 2008
189
I've been doing some Google searches to try to find some sort of prebuilt solution...

This device looks interesting. Charge Lithium ion batteries and act as a DC/DC converter to output a voltage that 12v vehicle devices want, plus also shut down so that the batteries cannot be discharged past the point where they are damaged. They have tiny ones like I would need, plus ridiculously high amp ones for forklifts, RVs, etc.

Inteli-Power® PD9100L Series
12- & 24-Volt Lithium Ion Converter/Chargers

https://www.progressivedyn.com/lithium_battery_charger.html

Though it lacks lithium battery pack temperature monitoring which would be a highly desirable hands-off feature.

Another negative is that it charges the entire cell stack as one unit and cannot do inter-battery series balance charging, or individual cell-fault detection. (This is accomplished with sense wires back to the charge controller which connect between each cell.)
 

Thread Starter

DMahalko

Joined Oct 5, 2008
189
Well, the following is not a perfect solution for their lack of battery overheat protection, but would probably work acceptably. Plus as a simple spring switch, can use several in series to monitor battery pack temp in several locations.

Snap Disc Thermostats, Safety Switch, Manual Reset, Open On Rise, 135F / 57C
White-Rodgers 3L12-135
http://www.mouser.com/ProductDetail/White-Rodgers/3L12-135


Difficulty: 120v AC 15A rating, no clue how to derate this to 12v / 13.6v DC, but will probably be fine for 100 watt load at about 8.3 amps DC.
 

Thread Starter

DMahalko

Joined Oct 5, 2008
189
... some more searching suggests I should look at "solar charge controllers".

Other keyword phrases to try:
  • lithium "battery management system"
  • lithium "battery protection" module/board
  • lithium "protection circuit" module/board
You have to know the lingo to find the correct technical solutions.

Difficulty: solar charge controllers with integrated AC charging input ability, is typically a huge thing used in home solar panel installations. That would not fit under my computer chair.


An example of the weird corners of Amazon that I didn't know existed:
https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias=aps&field-keywords=lithium+protection+circuit

(Next question, do I trust my house not burning down using a lithium battery protection circuit sold by strangely named companies I've never heard of, like Yeeco and ICstation? (Well, it is probably far safer than me trying to hand-build a similar protection board myself.))
 
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Thread Starter

DMahalko

Joined Oct 5, 2008
189
I have found yet another bit of lithium weirdness to throw into the mix, but may actually be safer and easier to deal with than nearly all other lithium / lead-acid substitution options I've been researching.

Stark Power - Deep cycle 12v lithium batteries
https://starkpower.com/product-category/deep-cycle-battery/?orderby=price

These are 12v lithium Ion batteries in standard sealed lead-acid battery sizes, and which include a built-in on-battery management controller. These are designed to be directly installed into a device meant to be used with lead-acid batteries and requires no changes to the lead-acid charger, which would normally damage lithium cells. The built-in controller also automatically prevents excessive discharging that would damage the lithium cells.

So, these don't need a special lithium charger, can use a cheaper charger intended for lead acid batteries.
 

MrSoftware

Joined Oct 29, 2013
2,057
A balance charger will charge the LiPo cells individually, this is what you need for a multi-cell LiPo battery. Many of them also include temperature sensors to stop charging if the battery overheats. Check out something like this:

https://www.amazon.com/dp/B01HXHGARY/ref=psdc_2234135011_t2_B00H9K8JY8

LiPO packs typically have two big leads for the load that are all cells in series, and they'll also have individual wires (much thinner) to each of the individual cells. This way the charger can charge each cell independently, but the load can draw from the entire battery at once.
 
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