Deep cycle Lead-Acid battery charger (48v)

Thread Starter

engineeringfront

Joined Dec 5, 2011
3
Hello everyone!

This is my first post here, so I'll briefly introduce myself: My name is Francisco, I am 22 years old and I live in Argentina. I am a technician in Aeronautics and I´m studying Aeronautical Engineering.

Together with 2 friends we set to build an electric motorcycle. We are designing the structure and we bought the motor (48V 2KW BLDC hub motor), controller and throttle from GoldenMotor in China (http://www.goldenmotor.com/). We also got 4 deep cycle lead acid batteries to power it at 48v DC.

The issue we are trying to solve now is the battery charging. We want to be able to charge the 4 batteries together (they will be connected in serial reaching 48V). Argentina power lines are 220V AC ~50Hz and we would like to find a solution that let us plug the motorbike directly to a regular power outlet (the charger will be mounted on the bike, so it should be compact and low-weight).

Non of us is really experienced in electronics, but we understand the very basics.

I will really appreciate any advice you could give us.

Thanks

Francisco Piaser
 

wayneh

Joined Sep 9, 2010
16,397
I don't have a full solution for you, but I encourage you to consider solutions that charge the 4 batteries independently instead of in series. This strategy will be far superior in extending the life of your batteries and ensuring an optimal charge in each.
 

Thread Starter

engineeringfront

Joined Dec 5, 2011
3
Hi wayneh thanks for your answer! I hope not to bother you too much with a few more questions. Can I charge them individually while they are still connected in serial? Also, will I need 4 individual chargers to do this? Thanks again!
 

SgtWookie

Joined Jul 17, 2007
22,210
Can I charge them individually while they are still connected in serial?
Yes, you can.

Also, will I need 4 individual chargers to do this?
You will need a charge controller for each battery. The charge controllers will need to be galvanically isolated from each other; otherwise you would wind up with direct shorts across the battery terminals.
 

Thread Starter

engineeringfront

Joined Dec 5, 2011
3
SgtWookie thanks for your reply! could you please provide me some information on how to build this charger? any reference circuit, or instructions on how to do it will be greatly appreciated.

Thank you very much for your assistance!!
 

SgtWookie

Joined Jul 17, 2007
22,210
First, you need to obtain the datasheet(s) for your particular batteries.

Although lead-acid batteries very generally have similar construction, the chemistry, actual construction and plate thickness have a lot to do with how fast you can charge them.

Discharging and/or discharging batteries rapidly leads to power dissipation in the battery, and the internal temperature increases, making it more chemically active, while at the same time reducing the voltages where it is considered charged and discharged. The temperature coefficient works out to be about -3mV/°C offset from 25°C per cell, or -18mV/°C offset from 25°C for a 12v lead-acid battery.

Here is a paper which discusses design trade-offs for switch-mode battery chargers:
http://www.ti.com/lit/ml/slyp089/slyp089.pdf
It's really intended for lithium ion chargers, but a large part of it is also relevant to lead-acid batteries.

You would not want to build a traditional type charger like was made some 30 years ago; it would be large, heavy, expensive and inefficient.

Unless you have at least intermediate skills in electronics and a minimum of an oscilloscope for test equipment, I would encourage you to purchase a commercial off-the-shelf solution, as you will have the item much faster, it will work straightaway, and probably won't cost you any more than if you built it yourself.

This is not a project for someone who is a novice in electronics, as the learning curve would simply be too steep.
 

SgtWookie

Joined Jul 17, 2007
22,210
Here is a product page on Texas Instruments' website:
http://www.ti.com/product/bq2031
The BQ2031 is a fast charge switching controller for lead-acid batteries.
You can control a number of parameters with it, and it provides for several different modes/phases of charging. It would be difficult to build that kind of functionality using something else.
 

Bazz

Joined Nov 22, 2011
10
Hi Sgt Wookie; looking at the Texas Instrument docs, there are a number of application notes for Li-ion batteries, and for single cell 2.2V lead acid applications, but I don't see how you take this to the next step of using it on a 12V lead acid battery. Did I miss something obvious :)

Bazz
 

SgtWookie

Joined Jul 17, 2007
22,210
Did you happen to look at the link I posted in reply #7? The bq2031?

That IC is a fast charge controller for lead-acid batteries. As far as the "2.2v single cell" -- the notes are written that way, as lead-acid batteries are available in various multiples of 2v, not just 12v.

The formulas for voltage-related calculations include "N" to factor in the number of cells in the battery you wish to charge.
 

bountyhunter

Joined Sep 7, 2009
2,512
If you can find some 12V (auto) battery chargers that are ISOLATED FROM THE POWER LINE you should be able to stack them and charge the batteries in series. Just hook each charger across each batttery. But, if they have a common internal ground, they will blow up.

Another option is to look on ebay for a used bench power supply rated for 60V/20A and use it to charge the string setting it to about 13.6V per battery, or about 54.4V. As mentioned above, charging in series is not the best approach, but it will work.
 

Bazz

Joined Nov 22, 2011
10
Did you happen to look at the link I posted in reply #7? The bq2031? .
Yes I looked at that but the 12V circuit is in one of the secondary links which I had missed. The PCB for that circuit sells for about $110 in the UK, so think I'll give that one a miss.
 
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