DIY Electric Car Forums banner
1 - 16 of 16 Posts

·
Registered
Joined
·
53 Posts
Discussion Starter · #1 ·
In my EV project I am trying to avoid lead batteries if at all possible. I have been using the hobby 3S LiPo batteries like you would use in RC cars and it works great, except right now I have no onboard charging. I just watch the voltage and swap out packs when it is getting low. Not practical, but not terrible since the range of the car is only 25 miles or so. With no power _anything_ in the car the only draw on the 12v system is the motor controller brake lights and turn signals.

Battery Technology Electronics accessory Electronic device Multipurpose battery

I have a 13.8v DC-DC converter but its not hooked up yet.

Question is: how can I get this DC-DC converter to charge my 3S LiPo pack when the high voltage is on, without overcharging it? Would just a BMS do the job for me? It just needs to provide voltage to the car until the contactor for the motor control closes, which is around 1 second after the key is switched on.
 

·
Registered
Joined
·
929 Posts
Unless you plan on replacing that LiPo frequently and / or constantly have a high risk of "unexpected thermal runaway" (boom bad) you need a charger that puts out precisely the right voltage for the pack, and knows when to stop.

And they don't last that long anyway, require specialist gear to monitor for EoL.

LFP has similar requirements, but is **much** safer. 13.8A happens to be a good setpoint for 4S, and you have more leeway for charge termination timing.
 

·
Registered
Joined
·
929 Posts
And terminology, House by definition means "not related to vehicle operations". Like screen gadgets, a fridge, fans and lights in the living space. For cruising boats, campers, van life, like that.

Usually 100Ah is a very small House bank, 800Ah medium.

No one uses LiPo for that.

LFP yes, Winston, CALB, Sinopoly, GBS large prismatics.

For your use maybe a Battle Born will do, as long as the limited C-rates are OK.
 

·
Registered
Joined
·
929 Posts
Now for the question :cool:

how can I get this DC-DC converter to charge my 3S LiPo pack when the high voltage is on, without overcharging it?
You need to stop charging when voltage measured at the pack hits between 12.3-12.6V

but that's only if you need to get to Full, which you don't, 12V will probably be OK in a pinch.

Just get a voltmeter and keep an eye on it.

But 13.8V is 4.6Vpc, so very dangerous.

Sure an HVC can cut off the converter, but risky to depend on cheap Chinese stuff.

BMS should be last-resort when the top layer fails.

> It just needs to provide voltage to the car until the contactor for the motor control closes, which is around 1 second after the key is switched on

What is the current load?
 

·
Registered
Joined
·
53 Posts
Discussion Starter · #5 ·
Thanks for the info and advice. I'll check on current load but should be like 2a most of the time (switching brake lights / turn signals to LED) up to 6a with the headlights on (which won't happen much if at all). I'll measure it and find out for sure.
 

·
Administrator
Joined
·
6,310 Posts
Hi Hummbug

You will find that I am somebody who enthusiastically overloads his batteries - and my advice to you is to take that Lithium Poly battery out of the car and NEVER put it back!

Lithium Poly is OK for RC cars and some people do use them for racing but is far far to delicate and unreliable for use in a car

What will happen is that one cell will fail - the DC-DC will overcharge the other two and it WILL explode
 

·
Registered
Joined
·
53 Posts
Discussion Starter · #8 ·
Thanks for the reply everyone. What if I used 3s3p 18650's instead? Same voltage situation. What would be the best way to safely charge the battery, and prevent over discharge?
 

·
Registered
Joined
·
53 Posts
Discussion Starter · #11 ·
My car is based on e-golf batteries which are lithium ion. So I am "all in" on this technology.

I am not a complete boob / N00b and just asking for honest advice here, and share the knowledge. I appreciate the advice and links!
 

·
Registered
Joined
·
2 Posts
In my EV, I use a 4s13p pack of Tesla 18650 cells from User Okashira from Endless-Sphere, who took apart a salvage Tesla and custom builds packs.

Max 16.8v Min 13.2v and a 2amp universal laptop charger set at 16v charges it to 16.4v, and I use a cigarette lighter voltmeter to monitor.

Incandescent bulbs might burn out faster, but all my lights are LED including dashboard. Nothing seems to have a problem with 16.4v, not even the aftermarket stereo. I enjoy getting extra electric miles by not powering a DC-DC converter.
(These packs can be applied to any vehicle, gas or electric, and MPG or eMPG will be higher until the pack drains down to 14.4v and the traditional alternator takes over. I've used it in my Prius and gotten higher MPG.)

https://endless-sphere.com/forums/viewtopic.php?t=67661
 

·
Registered
Joined
·
46 Posts

·
Registered
Joined
·
929 Posts
For a tiny order like that they may well be able to afford to back up a warranty for a few weeks after shipping.

Just be sure to be ready to test as above upon arrival.
 

·
Registered
Joined
·
46 Posts
Back in 2016 or so, I bought 9 Real Force PWN 100 Ah cells and have had good luck with them. Guess I'm lucky. Thanks for the heads up.
 
1 - 16 of 16 Posts
Top