EV battery pack

[chota.sanjiv]

Hi Guy,
I am trying to build a 48v li-ion battery pack for EV conversion  I will be using used 18650 cells for my project. Please help me with the following questions..

1. Can I use 5v boast converter for each parallel group to boast the voltage to 5V from 2.75 to 4.2 and then connect the output of these in series, this will help me avoid balancing them and also use max capacity of each parallel group.
2. I plan to embed li-ion charger for each parallel group (60x1860 in the pack itself, is it possible, what are your thoughts on this.

regards,
sanjeev

[anishkgt]

Sanjiv bhai A diagram would speak much louder than words. From what I understand I doubt it's possible. Charging and discharging would be an issue. What's the amp required for your EV ?

Lay down a diagram for what you intent to do.


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[hayatepilot]

Answers to your questions:
1. NO!
The losses will be too great and you don't need boost converters to use the maximum capacity of the batteries. The motor controller covers that part. It is essentially a step down converter to regulate the motor speed.
You just have to choose the motor and battery voltage so that it delivers the minimum desired power when the batteries are almost empty. If it's fully charged you get a higher maximum power, but that's not a bad thing and you can always limit the current in the motor controller.
Then you don't want to discharge lithium batteries completely on a regular basis because it will severely limit the cycle life of the pack. Always leave at least 20% charge left in the battery.

2. Yes you can use a seperate charger for each parallel group. This also balances the cells since each will finish charging at exactly 4.2V

But if you struggle with those trivial things then you should probably stay away from those high power electronics. We are talking several 100amps here!
Please be careful.

[CraigHB]

Converters get into big losses at high currents so that wouldn't be a good idea.  You don't see boosters off-the-shelf that can handle big currents like that anyway.  Plus it would be redundant, the motor controller handles regulation of power output regardless of input voltage.  As already mentioned, just need to design things so power output meets demands when the battery is in a low state of charge.

You didn't say what kind of EV, but a 48V pack would be something typical for an eBike or eScooter.  Probably would not get into hundreds of Amps with that, but still a considerable current draw at maximum outputs.  In any case there are safety considerations when working with Li-Ion batteries, so yeah, not something to be careless with.

You do need to provide a mechanism to protect against over-current and over-discharge of the battery pack.  These are safety considerations primarily, but otherwise severe damage to the pack can occur when limits are exceeded.  As stated already, you don't want to fully discharge the pack, 20% would be liberal so you can probably go a bit lower than that.

You may want to use high drain cells over the standard ones.  The capacity is somewhat lower, but they have advantages like lower operating temperatures and better longevity.  Of course they also have much higher drain limits.  Typically 10C compared to 2C.

These are pretty basic concepts in working with Li-Ion batteries.   If you're not already aware of these things you should read up on it.  This type of battery can be hazardous if not handled properly.  Mainly there's a fire hazard, but in some cases packs can explode when abused causing bodily injury.

[chota.sanjiv]

Hi Guys,
Thanks for the responses and safety concerns, I suppose I will have to use a BMS then. I have some more question related to the use of BMS and how it should work along with charging. I would appreciate your responses.

1. How does a BMS work along with a li-ion balance charger.
2. What should I do if i need to charge the cells while I am discharging like regen braking, disconnecting the discharge seems unrealistic while running.
3. How is the li-ion CC-CV curve maintained with a balance charger, I can understand for a single cell charger but here I am using 13 in series and not sure how to maintain a single CC-CV curve across all the parallel block.

My controller expects nominal battery pack voltage to be in 36v to 48v range and can pull 350amp of current, the battery pack voltage at full charge (4.2v) will be 54 and at safe cutoff (2.75v) will be 35.75v.

I am also attaching the connection diagram for the battery pack.

thanks,
Sanjeev

[CCitizenTO]

Then you don't want to discharge lithium batteries completely on a regular basis because it will severely limit the cycle life of the pack. Always leave at least 20% charge left in the battery.

Would be good to include a multi-stage low battery warning. First stage could warn them at 30% charge left. Second Stage could warn them at 20% they need to recharge ASAP.

2. What should I do if i need to charge the cells while I am discharging like regen braking, disconnecting the discharge seems unrealistic while running.

I would say store the energy from braking with some sort of regenerative braking system in capacitors because you will probably want to use that energy fairly quickly rather than trying to use it to recharge the batteries.

[chota.sanjiv]

My initial post for using DC-DC converter for based upon this post, not sure if I understood correctly what he is taking about.

http://liionbms.com/php/wp_redistribution.php

[CraigHB]

How is the li-ion CC-CV curve maintained with a balance charger, I can understand for a single cell charger but here I am using 13 in series and not sure how to maintain a single CC-CV curve across all the parallel block.

It's possible to charge a pack as a single unit using only two wires, but that's actually not the correct way to charge a Li-Ion battery pack.  What happens in that case is cell imbalances occur where one cell may surpass the 4.2V limit (typical) where other cells are still below that.  It gets progressively worse over more charge cycles until it can just flat out kill the pack.

There are a couple ways to approach balance charging. 

The hobby style chargers that are made for LiPo packs use a method where once the first cell in the pack reaches terminal voltage it discharges that cell through the battery's balance connector until the other cells catch up.  That's actually not an ideal design since the balancing phase can add time to the charge cycle.  If cells are heavily out of balance it can add a lot of time.  You're also putting current in only to take it out again which is inefficient.  The reason it's done that way is to support the high charge rates this type of pack can accept.  A high current can be put on the pack through two wires where the balance connector trims the voltage on each cell.

The way it's done ideally like in Laptop computer packs is each cell or block of parallel cells is handled as an independent unit.  The charger controllers are stacked so each each cell or block of parallel cells charges independently of the others.  However, this type of controller typically does not support very high rates, usually no more than a couple Amps which can result in long charge times for high capacity packs.  If you want to charge faster you may need to look at the way it's done with hobby chargers.