Voltage regulator rectifier for LiFePO4 battery

[JohnDB]

This might also apply to changing the lead-acid battery on your tractor to LiFePO4, but for me its about my Honda TRX500 quad bike.
Specs for LiFePO4 automative 12V batteries say the charge volts needs to be 14.6VDC or less to avoid damaging the battery - even if the battery has a built in battery management system. The quad bike goes up to 15.5VDC, is there a simple reliable way I can reduce the 15.5 V to not more than 14.6? Charge current 400W at 5000 rpm.
Charging system specs:

Aftermarket VRRs are available to convert to LiFePO4 but I'm keen to avoid the hundreds of dollars cost.
Thanks for your help.

 

[GreensvilleJay]

Simple, easy way is to add 1 diode in series between the alternator and the battery. Regular silicon diode 'drop' about .7 volts under a load. Get diodes rated for 20 amps (more is better ).
You 'may' need to add a heatsink to the diode IF it gets warm.

 

[ctfjr]

Simple, easy way is to add 1 diode in series between the alternator and the battery. Regular silicon diode 'drop' about .7 volts under a load. Get diodes rated for 20 amps (more is better ).
You 'may' need to add a heatsink to the diode IF it gets warm.

looks like he would need 2 in series to get below 14.6v, but I like the simple solution

can't get much cheaper: amazon
I think the heat problem for the diode is going to be, well, a problem

.7 volt drop * 1amp charging current would be .7w dissipated in that diode. If you are charging at a higher rate, like 5 amps them you are going to have some real heat produced (.7*5=3.5w)

The best bet would be to find diodes with a built in heat sink. Since you need 2 diodes in series I'd bet a full wave bridge rectifier would be perfect.
Like this: amazon bridge

 

[GreensvilleJay]

yup, a GREAT, EASY solution...... even comes with a handy mounting hole !!!!!
+ve 1/4" tab to the battery
-ve 1/4" tab to the alternator
simple, effective, cheap and reliable

 

[JohnDB]

Hi GreensvilleJay, ctfjr, thanks so much. Looks like a great idea. I'll put a meter on the charging circuit and see what it actually delivers (manual says "up to 15.5V") to determine whether I need one diode or 2.

If I go the bridge rectifier option, how should that be wired in? Like this (to get 2 diodes in series)?

[Edit 1 - I didn't see GreensvilleJay's post above until after I'd put up this one. It might have answered my question here]
[Edit 2 - yes it does... I assume I bridge the 2 AC terminals too. Thanks!]

 

[ctfjr]

Hi GreensvilleJay, ctfjr, thanks so much. Looks like a great idea. I'll put a meter on the charging circuit and see what it actually delivers (manual says "up to 15.5V") to determine whether I need one diode or 2.

If I go the bridge rectifier option, how should that be wired in? Like this (to get 2 diodes in series)?

View attachment 132821

[Edit 1 - I didn't see GreensvilleJay's post above until after I'd put up this one. It might have answered my question here]
[Edit 2 - yes it does... I assume I bridge the 2 AC terminals too. Thanks!]

no, that won't work

try this

You will have 2 paths through a pair of diodes in each. Not a problem with two paths, you will just get a voltage drop equal to the pair with the lowest forward voltage drop (insignificant here). As a bonus if 1 diode leg fails it will 'fall back' to the other.

So your voltage regulator will connect to the "-" output of the bridge and the battery (+) will connect to the "+" output of the bridge.
Just 2 wires. NO ground wires.

 

[JohnDB]

no, that won't work

try this

View attachment 132869

You will have 2 paths through a pair of diodes in each. Not a problem with two paths, you will just get a voltage drop equal to the pair with the lowest forward voltage drop (insignificant here). As a bonus if 1 diode leg fails it will 'fall back' to the other.

So your voltage regulator will connect to the "-" output of the bridge and the battery (+) will connect to the "=" output of the bridge.
Just 2 wires. NO ground wires.

Brilliant, thanks!

 

[Oil pan 4]

Simple, easy way is to add 1 diode in series between the alternator and the battery. Regular silicon diode 'drop' about .7 volts under a load. Get diodes rated for 20 amps (more is better ).
You 'may' need to add a heatsink to the diode IF it gets warm.

I tried that it doesn't work. At low load the forward voltage drop can be 0.1 volt or less.

 

[GreensvilleJay]

Add a 'burden' resistor...though is means some power is always being wasted.
What you want is a bit of a challenge some 'black box' that drops 1 volt while passing up to 8 amps.
Not impossible, one solution is to feed the alternator voltage into an upconverter then into a fixed regulator. Assuming 10 amps is the maximum current, can be done with 'off the shelf' parts.

12a - Temu Canada

12a current 19v on Temu. , & .

www.temu.com

a possible starting point ? don't have one but might work ??

another version, fixed Vout though

was discontinued - Temu Canada

8v 40v to 12v 6a 72w at the at Temu. Check reviews and See 's new of , & . .

www.temu.com

would have to have them, battery to test on the bench......to prove 'yes or no'.


another option, use a LDO regulator like a MIC29xx series. Needs 2 caps, pot and heatsink...good for 7.5 amps .Good news is you're only looking at about 10 watts to get rid of,so small heatsink.

 

[WI_Hedgehog]

Buy a better LiFePO4 battery. We replace AGM with LiFePO4 all the time in the motorcycle world and it's not an issue when the battery is built well and has a BMS designed for automotive charging systems. Personally I run an EarthX aviation battery (with redundant internal electronics) due to the expense of the bike, but any solid automotive use battery will do--they're designed for it. Charging systems are standardized across the industry, if you're trying to modify a charging system to fit the battery you're using the wrong battery.

The only issue I've personally had is putting a fully charged LiFePO4 battery into a vehicle and popping the main fuse due to the higher LiFePO4 resting voltage and LiFePO4's incredible responsiveness to surge current; pre-charging the vehicle circuits with a lead-acid battery, or using a load resistor to charge the vehicle circuits more slowly, or using a LiFePO4 battery at 50% charge resolved that.

 

[lugbolt]

the simple solution is to put a AGM or FLA battery in, and be done with it. Toss a battery tender on it if it sits a lot (most ATV's do sit quite a bit).

you can buy regulators in different configurations but you may have to change the connector. I've used Yamaha regulators on Mowers in the past to to get the voltage up a little. Just takes some wiring, and a connector.

depending on the regulator you can also "lie" to the sensing circuit to increase or decrease the charge voltage.