Electronic Bike conversion with Lawnmower Battery? - HELP NEEDED

Thread Starter

griffin_designer

Joined Nov 19, 2023
3
Hello Everyone, I am really new to electronics and I am struggling to figure out how I would be able to use my GreenWorks 40 volt 4 amp hour battery on a 48 volt 1000 watt electric bike conversion kit. I have looked for a step up converter from 40volts to 48volts but I was only able to find one on AliExpress.

I don't know if the system will still work with a 40 volt battery or do I need to figure out a way to step up the voltage. My other concern is the quality of the Aliexpress converter and if it will go bad or start a fire. Please help me out if you are knowledgeable on this topic. Thanks!
 

Papabravo

Joined Feb 24, 2006
21,029
Sounds like a bad idea. When you step up a DC voltage you reduce the available current output. the reduced current output will noticeably degrade the experience of riding the electric bike.

Amp hours is a measure of battery capacity. It has nothing to do with how much current is available to start or run a motor.
 

MrChips

Joined Oct 2, 2009
30,518
Don’t use a step-up converter. You are not going to gain anything by going from 40V to 48V.

Also, do the math.
1000W translates to about 20A at 48V. If you use the ebike at 1/5 the power, i.e. drawing 4A, the battery will theoretically last for 1 hour. In reality, you will have to peddle after about 15 minutes.
 

Thread Starter

griffin_designer

Joined Nov 19, 2023
3
Thank you for responding, Would it be better to go with a 36 volt system 500 watt converstion kit and use a step down converter or no converter at all?
 

Thread Starter

griffin_designer

Joined Nov 19, 2023
3
Don’t use a step-up converter. You are not going to gain anything by going from 40V to 48V.

Also, do the math.
1000W translates to about 20A at 48V. If you use the ebike at 1/5 the power, i.e. drawing 4A, the battery will theoretically last for 1 hour. I reality, you will have to peddle after about 15 minutes.
Thanks for your response, one question I have a second 40volt 4 amp hour battery would it be a good idea to run them in series so that I get 8 amp hours of battery?
 

MrChips

Joined Oct 2, 2009
30,518
Try and match the battery voltage to the bike system. Don’t use any converter.

Putting batteries in series add the voltage.
Put batteries of equal voltage and type in parallel to add current.
 

MisterBill2

Joined Jan 23, 2018
17,885
I suggest looking at the specifications of the battery recommended to go with that controller, and comparing the specs with that green works battery that you have.
 

schmitt trigger

Joined Jul 12, 2010
844
Try and match the battery voltage to the bike system. Don’t use any converter.
Exactly!
As a person who also owns an e-bike, I can strongly suggest not to attempt to kludge a battery.
Both for range and safety reasons, you require to purchase the correct 48 volts battery.
 

Ya’akov

Joined Jan 27, 2019
8,975
My experience with Greenworks batteries suggests they would not be a good choice. They appear to be of relatively low quality and capacity cells. While they might have moved to better cells, the batteries I have (4 40V and 2 80V) are very expensive no more than average devices.

There are eBike specific batteries from companies like Bosch that I would much sooner trust. In fact, due in no small part to the batteries, we no longer have any Greenworks equipment and have replaced everything with Makita.
 

MisterBill2

Joined Jan 23, 2018
17,885
The statements by "Y" back up my comments based on observation of other folks experiences, and requests to "see if I can fix the charger that no longer puts a full charge in the battery." That is always an unhappy situation when the charger is actually working but the battery has lost much capacity while still indicating a full charge.
 
Define correct.

As long as the voltages and currents are all the same it doesn't make a difference.
Yes and no.
During a heavy workout, the battery becomes very warm. And I mean really warm.
I would not locate a non-thrustworthy battery between my legs and a few inches from my groin.
This is not idle speculation. A thermal even actually happened to a member of the bike club I belong to. And guess what, it was on a modified bike. Fortunately he reacted quickly and bailed out. He only bruised his left side.

Before anyone starts arguing with me, I am not discouraging bike modifications. But don’t mess up with the battery.
 

Ya’akov

Joined Jan 27, 2019
8,975
Before anyone starts arguing with me, I am not discouraging bike modifications. But don’t mess up with the battery.
Not arguing but—maybe “specifying”.

I completely agree with this sentiment but would say it can be OK, if it is approached with the right degree of rigor and some excess of safety measures. The rigor includes thorough education about battery failure modes and their root causes (this can be in an autodidactic vein, if someone is so inclined, but must be honest. That is, if you don’t understand something, seek out expert clarification).

Rigor also demands extensive static testing before putting a battery on a bike and counting on it not to become a flamethrower. Unfortunately, one strong motivation to homebrew a battery is cost saving. This tends to make safety far less certain thanks to using cheaper or more suspect (reused cells) parts.

This doesn't mean you can’t create a less expensive battery for yourself but it does mean knowing where you can safely save money and where cheaping out means a seriously elevated risk.

One more thing: it is possible to build a battery with “shielding” to protect the user against the dangerous results of a thermak runaway. You can’t contain a thermal event, that‘s just building a bomb—but you can direct the flames by making the faces of the battery that are most likely to be adjacent to the person relatively impermeable to the boiling electrolyte and relatively heat proof so they maintain their integrity in the face of heat and fire inside the battery.

As a compliment to this shielding, it is necessary to provide a route for safe venting, away from the person as much as possible. In theory, it seems possible one could even mitigate the size and burning of the electrolyte stream forced from the battery.

The question is, can you derive enough advantage in your particular application to make the necessary level of rigor and effort worth the time and trouble? It doesn’t come free, and it requires vigilance. For motivation, watch a vape pen malfunction video—consider that is a single 18650, then multiply by the size of the battery you might want to build. Is it worth the risk?
 
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