Go Back  Bike Forums > Bike Forums > Electric Bikes
Reload this Page >

Anyone know how I can charge my battery from another larger battery?

Notices
Electric Bikes Here's a place to discuss ebikes, from home grown to high-tech.

Anyone know how I can charge my battery from another larger battery?

Old 02-12-18, 08:30 PM
  #1  
Alligator
Full Member
Thread Starter
 
Alligator's Avatar
 
Join Date: Sep 2011
Location: Minneapolis, MN
Posts: 200

Bikes: Too many. I’m constantly selling and buying new bikes.

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 90 Post(s)
Liked 36 Times in 24 Posts
Anyone know how I can charge my battery from another larger battery?

I’ve had a computer running 24/7 for a number of years off of a large lithium battery and a solar panel. It’s worked well, but I no longer need the computer and now I’d like to repurpose the battery and solar panel. It charges during the day when I’m at work, and I’d like to see if I can charge my 13.5 Ah ebike battery in the evening from the solar-charged battery.

Does anyone know if there is a DC-to-DC charger out there? The large lithium battery is 12-14 volts (IIRC). I’d have to step up the voltage since my ebike battery is 52 volts.
Alligator is offline  
Old 02-12-18, 09:27 PM
  #2  
unterhausen
Randomhead
 
Join Date: Aug 2008
Location: Happy Valley, Pennsylvania
Posts: 24,363
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked 3,664 Times in 2,497 Posts
I always just run a ac inverter off of the battery, since they are so cheap. Then your current charger will work.
unterhausen is offline  
Old 02-13-18, 02:13 PM
  #3  
Khb
Full Member
 
Join Date: Apr 2017
Posts: 203

Bikes: Stromer ST-1; Gary Fisher SAAB edition; Dahon Speed D7; Motobecane Grand Touring 1972

Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 38 Post(s)
Likes: 0
Liked 2 Times in 2 Posts
It is typically more efficient to convert DC to DC directly rather than inverting ... but finding a plug ready unit may be difficult (how good are your DIY electronics skills?)
Khb is offline  
Old 02-13-18, 03:05 PM
  #4  
Doc_Wui
Senior Member
 
Doc_Wui's Avatar
 
Join Date: Jul 2015
Location: Chicago Suburbs
Posts: 1,395

Bikes: GT Transeo & a half dozen ebike conversions.

Mentioned: 5 Post(s)
Tagged: 0 Thread(s)
Quoted: 339 Post(s)
Liked 268 Times in 189 Posts
Lithium batteries like to be charged from a constant current supply and they want it to shut down at the proper rest voltage as the current decreases. For my 13S battery, that would be 54.6 volts.

You might get your DC-DC converter to output the right voltage, but can you have it shut down the current to prevent overcharge? I think Unterhausen's advice for the regular lithium charger on an inverter is smart. A loss in efficiency isn't a big deal here. Perhaps it would be in a situation where every electron had to be hoarded.

The charger will probably run on DC if it's high enough. A 12-72 DC-DC converter might suffice instead of an inverter.
Doc_Wui is offline  
Old 02-13-18, 04:15 PM
  #5  
Alligator
Full Member
Thread Starter
 
Alligator's Avatar
 
Join Date: Sep 2011
Location: Minneapolis, MN
Posts: 200

Bikes: Too many. I’m constantly selling and buying new bikes.

Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 90 Post(s)
Liked 36 Times in 24 Posts
Does anyone know the amount of current a standard charger draws? I'll have to read up on this before I plug it into an inverter and fry the inverter. However, the good news is that I'm not really worried about efficiency. After all, it's being charged up by the sun! A little loss here and there won't make much of a difference to me.
Alligator is offline  
Old 02-14-18, 11:28 AM
  #6  
Rick Imby
Senior Member
 
Join Date: Sep 2015
Posts: 663
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 239 Post(s)
Likes: 0
Liked 3 Times in 3 Posts
Most of the ebike battery chargers are 2 or 3 amps. I agree your current charger hooked to an inverter would be the safest way to recharge.

----Pretty cool idea to use a solar panel to recharge a static battery and then recharge your bike.
Rick Imby is offline  
Old 02-18-18, 09:29 AM
  #7  
unterhausen
Randomhead
 
Join Date: Aug 2008
Location: Happy Valley, Pennsylvania
Posts: 24,363
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked 3,664 Times in 2,497 Posts
do ebike batteries typically have balancing circuitry built in, or is that in the charger? In any event, a dc-dc inverter really involves going to AC. I don't think going to 120vac really introduces enough inefficiency to worry about, particularly when the power is coming from solar. Relatively high power dc-dc converters aren't cheap, particularly for voltages that aren't in very common usage, and ebike battery voltages aren't common at all. DC-DC converter design is still a relatively difficult design task, you don't see that many diy projects involving converters.

I think what might be possible for someone with electronic sophistication is to inject DC after the input circuitry/rectification circuitry of the charger. You would need a boost converter to raise the voltage to some level above the voltage of the battery you want to charge. It occurs to me that the circuitry to do this is included in an ac inverter.
unterhausen is offline  
Old 02-18-18, 10:26 AM
  #8  
dilkes
Senior Member
 
Join Date: Jun 2013
Posts: 224
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 44 Post(s)
Likes: 0
Liked 1 Time in 1 Post
Originally Posted by unterhausen
do ebike batteries typically have balancing circuitry built in, or is that in the charger?
Yes, one of the tasks of the BMS in the battery pack is to manage the balancing.
dilkes is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Bigbadjohn
Electric Bikes
1
09-14-19 06:30 PM
Tuchango
Electric Bikes
2
01-02-15 12:09 PM
chillspike
Electric Bikes
16
05-08-14 07:26 PM
mrmagoo
Electric Bikes
3
03-26-14 05:08 AM
John Phoenix
Electric Bikes
1
11-13-10 12:46 AM

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.