Are two batteries better than one?

Desertprep

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I have decided to buy a new scooter, rather than a used. The deal breaker was the brakes. The new ones all have disk brakes - many of them pretty good, while the used ones all have a drum assembly that is rather finicky in wet and cold weather, at best. I will be buying the scooter sans battery in order to get (what I think is) a better battery. There is room under the seat for 1 60 amp LiFe battery or 2 30 amp LiFe batteries (2 batteries will require some minor mods). My reasoning for considering 2 batteries is that I will be able to charge them both at the same time with different chargers and, hopefully, get the batteries charged in half the time. Is my thinking correct? If so, what do I have to consider re: wiring? The batteries will be removable but also hope that they can be charged within the scooter.
 
You can charge a single battery with two chargers just as easily.
 
I suppose you are thinking parallel packs?
This is a pretty dangerous proposition unless done right. I would say it's not worth it if you can have a single pack with the same capacity.

Paralleling chargers is safe if the combined charge current is appropriate for your pack. Do that if you just want to charge faster.

If you thought it's better to charge the packs separately when you use two packs. No, the opposite is true. If you keep them parallel then at least the packs are in balance (not the cells and therein lies the problem).
If you charge them separately and then connect them, sparks might fly.

If you are thinking serial strings, then none of what I said is relevant. Do it, it's a good idea.
Charging serial strings in sections is basically incompatible with good and safe BMS topologies. We should talk about that...
 
What do you mean by parallel charging? It would be ideal if I could recharge a 72v 50a battery pack in 4 hours - if I can do so on a regular basis, without shortening the life of the battery. Would a 5C rated battery tolerate this? My 48v 10a Life battery recharges in about 4 hours.

Note: motor will be 72v, 1000 watt
 
Sure a 5C 50Ah rated battery can be charged with 5x50A! Disregarding the CV regime, that would be charging in 12 minutes.
(Don't mix up C-ratings for charge and discharge. A lot of high capacity cells are only rated for 0.5C or 1C charge rate.)

But 1C would still mean up to 80-90% in one hour.

High current chargers are large, expensive and can require more input power than a single phase wall outlet can supply.
That is why even the most powerful single phase chargers are usually only 3.5kW. (16A at 230V)

For example, this really nice charger can only charge your 72V battery at 40A.
http://www.tccharger.com/english/Product/T39/57.html (You probably can't use this particular charger unless you have a BMS that supports the TC CAN protocol. I just took that as an example as it is a commonly used charger in OEM bikes.)

Without knowing your setup really, I would guess your C-rating is incorrect.
For practical purposes, aim for 1C fast charge unless you know exactly what your batteries can take. And since you likely don't want to invest in three phase or CHAdeMO fast charge you would be limited to that range anyway.

PS: In any fast charge regime, be sure to play it safe. A good BMS with battery temperature monitoring is essential. :wink:
PPS: Charge with 0.5C (25A) and you will be safe. And should still stay within your 4h range. And your battery will last longer.
 
5C discharge does not mean it can take a 5C charge. especially not pouches. they tend to explode when you do that...

always charge as slow as you can get away with. if you use it as a daily driver then just use a charger that takes 4 hours or more. then you will have a full ride every time you need to drive.
 
TilmanBaumann said:
If you are thinking serial strings, then none of what I said is relevant. Do it, it's a good idea.
Charging serial strings in sections is basically incompatible with good and safe BMS topologies. We should talk about that...
Serial strings? You mean like Captain Crunch or Special K?? Just kidding :) I am assuming that you mean one battery is 24v and one is 48v, or each of two is 36v, for a total of 72v? If I understand you correctly, it is a good idea to use serial strings but they should be charged in serial.

So if two batteries of different voltages are used in serial, can they be mounted in separate places on the scooter - as much as 1.5 feet apart?
 
pengyou said:
I am assuming that you mean one battery is 24v and one is 48v, or each of two is 36v, for a total of 72v? If I understand you correctly, it is a good idea to use serial strings but they should be charged in serial.
Basically. If you series packs with the same type of cell type it's fine. Just, of course, use one BMS and not one per pack.
My problem is, the more information I give you the safer you feel about doing something really dangerous if you misunderstand me. I want to avoid that. Lithium Ion batters are scary dangerous if used wrong.

Let me try to package it.

Parallel connection of cells is safe and good. But you MUST use cells with the same voltage. You can mix different capacities.
Series connection of cells is good. But here it matters that the cells are mostly equal in capacity and power. (BMS balance those cells and react to dangerous over or undervoltages due to imbalance)

Packs are usually a hybrid of the two.
You probably came about ?S?P nomenclature before?
A 10S4P pack consists of parallel bundles of 4 cells which are connected in series. Resulting in say 4.1Volt * 10 == 41V.
Those are first principles.

What is absolutely forbidden is to build the same pack by connecting 4 series strings of 10 cells :!:

When you are trying to combine packs, you almost inevitably run afoul of those first principle rules unless the packs are identical.
Say you have two of those 10S4P packs. Similar age and cell capacity.
It is totally fine to connect those packs in series to create effectively a 20S4P topology.

What you can't do is to combine wildly different types of packs with different capacities. The cells will get unbalanced during charging and discharging. The BMS might prevent a fire. But they usually can't balance them strong enough to avoid the misbalance. And balancing happens by burning off excess energy. Not very useful to do unless you really can't avoid it.

If you parallel packs you have to follow a ton of safety rules to even have a chance of doing it safely. Definitely stay away from that.

And in the context of charging the packs. If you parallel the chargers, they combine power and it's totally safe.

You might be tempted to attach one charger to one pack of say a string of two packs in series. It looks like a good idea. But you are fighting the BMS in terms of balance and you deny the BMS the capacity to protect the battery by disconnecting the charger.

But let's get back to actually helping you particular.
What types of packs to you have? What do you want to do and what do you want to achieve.
I'm still not sure about the motivation behind your question.
 
Thanks! I am somewhat familiar with the terminology you are using. I have been researching this topic for a number of years, but every time I get close to actually buying something, something falls through - usually $$. This time, because of where I live, the scooter/battery will pay for itself in about three months compared to what I am spending on taxis. What I am hoping to do is to put a large (50+ amp, 72 v) battery in a scooter that probably doesn't have enough room in the original battery box for that size battery, hence the idea of splitting up the battery. Given your concerns - much appreciated, by the way, I am going to devote more time to looking for a scooter that was designed for such a large battery. I have not found one yet but have been told that they are out there. The other reason I was thinking of splitting up the battery back is to help lower the center of gravity. If I could put a 25 amp 72v battery in the original battery location and then put one 15 amp 72v battery on each side of the rear wheel in rigid saddle bags, I think the scooter would handle much better, and it would still likely leave me enough room to put my helmet under the seat when the bike was parked. I know that it is only relocating 30 pounds or so, but I have found that 30 pounds of groceries on a bike makes a big difference in the handling, depending on where they are placed...and then there is the issue of how long it would take to charge...
 
So 72V ~50Ah is what you are after, that is 20S and if you use 18615 cells roughly 20P.
10*3.5V = 72V (nominal. 84V when full. If you want to achieve 72V max then use 17S)
2 500 mAh * 20 = 50Ah
(Or did you actually mean Amps not Amp/Hours? Then that is achieved with a lot fewer cells in parallel. like 4-6 cells depending on what cells you use and how much you want to thrash them. Use as many as you can physically fit I would say)

Building two packs that together achieve that is no problem. (Use ONE 20S BMS though!)
Say 10S20P + 10S20P. Or 5S20P + 15S20P.
Just use packs with the same or very similar cell types.


What you are describing in your response is exactly the more dangerous parallel strings scenario!
A lot of people do this. Most of them probably haven't burned their house down yet. :lol:
OrionBMS has a good write-up why parallel strings are a bad idea, even if you get clever about it.
https://www.orionbms.com/general/using-parallel-strings/
The issues and solutions are not OrionBMS specific.

But I'm preaching and not doing. I am myself are building a parallel pack configuration. :oops:
But I'm making damn sure that I make the lest unsafe and convenient compromise that works for me.
IMG_20180928_105023.jpg
Here a simple sketch of my approach.
I basically included bridges between the remote cells, effectively paralleling all the cells of one voltage.
But the issue with that is that a cell failure can overwhelm the current carrying capacity of this bridge (that is why I draw it fat. I use a thick wire) and if that is the case the BMS cannot detect the cell failure.
I don't like that solution one bit. But I'm stuck with the modules I have.

The safest parallel strings setup is for sure one with a BMS and charger per pack and a selector switch that allows switching between the packs. That isolates the packs from each other.

TL/DR;
ONE virtual battery split into multiple physical sections is always better than multiple batteries
 
Thank you! That is very helpful. It looks like, if I am going to do this, most likely will have to do it myself. Even though I am a novice, I don't think I would trust the shops around here to be so conscientious building a battery like that.
 
Building a pack from scratch is fun and not very difficult. :D

But I would not dismiss your local sources. I'm sure you can find a good manufacturer.
 
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