DIY 12v SLA to Li swap?

[helpfulguy]

Hello,
Was thinking about what is needed for a 12v starter battery replacement.

Current:
Peak 60A to get motor turning.
Sustained ~30A while turning.

Output voltage regulation
4s regulate down voltage to 12v. (what part)
3s regulate down or boost voltage to 12v. (what part?)

Low voltage cutoff
What part?

12v-input charger
4s http://www.hobbyking.com/hobbyking/store/__14633__HobbyKing_8482_E4_Balance_Charger.html
3s http://www.hobbyking.com/hobbyking/store/__7637__Turnigy_12v_2_3S_Basic_Balance_Charger.html

Battery pack:
What part? 5Ah would be enough, I was thinking hk hard case lipo but I dont like lipo, especially not close to oil and gasoline (sorry for blasphemy).

Circuitry:
SLA have input/output in the same leads, I understand how this works for SLA (overvoltage = charge) but in li i have no clue :/


Is it better to just get a factory produced sla-replacement li-pack for $80? But how fun is that?

 

[Nobuo]

if you want to use lithium-ion chemistry cells you should use 4s (16.8V max) because the voltage sag. 3s (12.6V max) would be not enough if they are not almost fully charged. I don't think you really need a different regulator for discharging, the same it work yet installed with SLA (14.7V max) should work with lithium.

for standard NCM/NCA lithium-ion 18650 cells you should use at least 6p (~60A max continuous)

if you use samsung INR18650-25R you can use 2p minimum

If you use LiFePo4 cells, just don't worry to much about current discharge, (3C to 10C)

the battery should have a BMS installed for the cutoff protection.

**Important: Lithium-ion based cells doesn't like to be fully charged and with a float charging like SLA cells stand healthy. Lithium-ion cells die in about one year if they are continuously 100% charged.

LiFePo4 cells are much more indicated for a car starter.

The problem with any lithium based chemistry is the charge voltage with the alternator, I don't really know how could be managed to charge lithium cells as they need (cc and cv phase), I'm sure others will explain what is needed to add.

 

[999zip999]

What do you want to use these battery for ?

 

[dmwahl]

What do you want to use these battery for ?

...12v starter battery replacement.

Definitely LiFePO4 is going to be simplest, 4 series cells will have an optimum charging voltage of 14.4-14.6V, while a lead acid battery will be around 14V. Since the alternator will put out about 14V, you'd be able to charge your LiFePO4 to ~3.5V/cell, which is close enough to 100% to be good enough.

The question should be asked though, why replace the lead with lithium? Weight savings? Fun project? You're unlikely to realize any cost benefits from it. From the current levels you shared, I'm guessing this is a scooter, motorcycle, or something relatively small.

 

[r3volved]

Here's a starter battery from John in CR: http://endless-sphere.com/forums/viewtopic.php?f=14&t=64748

I think Doctorbass has a whole thread about his starter battery but I can't seem to find it...he has a video and mention in this thread though: http://endless-sphere.com/forums/viewtopic.php?f=14&t=67813