Wednesday, 27 April 2016

Thorpy's Guide to Buying an EBike Battery

Thorpy's Guide to Buying an EBike Battery - Intro

There is only one thing worse than shopping for shoes with your wife or girlfriend, and that is buying an ebike battery. The variety of shops to buy from is large with branded systems (think Bosch) costing a small fortune, and cheaper "brand x" (i.e. unbranded) batteries still costing a painful amount but having no warrantee or support of any kind.

So what to do? I have been looking at batteries for around 2 weeks on and off, and I have found the choice a difficult one to make, mainly because lithium batteries are so expensive, and I am not used to spending that much money on bicycle components.

A good lithium based battery can cost upwards of £250 ($400), even if purchased direct from China, so its worth taking time to consider. Expect to pay a minimum of £300 for a quality battery purchased in the UK.

Thorpy's Guide to Buying an EBike Battery - Capacity (AH)

Summary: Bigger is Better

The capacity of a battery (think range or size of fuel tank) is normally given in Amp Hours (AH), typically the bigger the capacity of an ebike battery the more you pay. But before you spend a fortune on getting the biggest battery you can find it is worth considering how far you need to go.

Luckily for us somebody have developed an ebike range calculator for using to determine capacity required. Just fill in the boxes, and you will arrive at your required AH capacity. For me that was 15 AH.

This required capacity relates to a 48 volt system. If you did the same journey with a 24volt system you may require double the capacity.

I am not too hot with electronics so just use the calculator and you will soon realize what is going on!

Thorpy's Guide to Buying an EBike Battery - Voltage (V)

Summary: Must Match Existing if Replacing - Choose 48v if building from New

As mentioned above if you run a lower voltage you will require a higher capacity battery to travel the same distance, this is because a lower voltage ebike motor will use more amps, than a higher voltage motor to do the same amount of work.

A low voltage motor is probably OK in  flat areas where is does not have to work very hard, but if you require lots of power for hill climbing etc. the current flowing through a low voltage hub may damage the motor. The lower the voltage the higher the current needs to be to provide the same energy to the motor.

For example my 24v 500w motor draws 30A when climbing. The wires get warm, the motor is hot to touch. If you consider a vacuum cleaner runs on 13A or less, we can start to see that a high power motor at a low voltage is not very practical.

So if like me you wish to travel 20 miles over hilly terrain you will likely need a 15 AH battery if you are running a 48 volt system, but 30 AH if you are running a 24 volt system.

If you are building bike from scratch you can choose whichever voltage you think suits best (36v is the most widely used), 48v  for heavy going.

If you are replacing a battery you will have to stick to the same voltage, although some good quality motors can be run at higher voltages. But without getting too technical, just replace with a battery of the same voltage.

A final word. It does not do a battery any good to run it down to flat every time you use it. So buy a battery a little bigger then you need. For Sealed Lead Acid Batteries you should only use half of the marked capacity! For most lithium batteries 80% - 90% is OK. Yet another reason why lithium batteries are so much better for ebikes.

Thorpy's Guide to Buying an EBike Battery - Types of Battery (Chemistry)

Summary: For Reliability Choose LiFePo4

Most ebike use some type of lithium battery. Lithium batteries hold lots of power for their size. Although if you are on a very tight budget you might consider sealed lead acid batteries, but these are very heavy.

For proven track record a "LiFePo4" Lithium Battery will be your best bet, although there is now a growing number of  "NMC" Lithium Batteries coming on the scene. These "NMC" batteries are lighter although they are likely less reliable over longer time periods.

Thorpy's Guide to Buying an EBike Battery - Manufacturers (Brands)

Summary: For Value Choose Samsung for Reparability Choose Headway

If you are buying an ebike battery choose known manufacturers. To be clear this is not for the battery itself, it is the "cells" within the battery.

As above; a battery is composed of many small cells, and you should choose a known manufacturer. For example you might find the battery described as 48v 10ah Samsung. If the battery does not include a known manufacturer in the description then it is probably made with cheap no name cells that may not last very long (although you might be lucky!). Look for:

  • Samsung
  • Sony
  • Panasonic
  • Headway
Most cells are 18650 type this number relates to the shape of the cell. We are used to describing cells as AA or AAA or even (gasp) LR6. 18650 fits in with these names. 

Thorpy's Guide to Buying an EBike Battery - Output (A)

Summary: Divide the motor wattage by the voltage and multiply by 2.5 for required.

You should check the battery's output is OK for your motor. We are looking for an Amps (e.g. 30A) figure here. A normal 24v 250w motor should draw about 10A, but in reality this could be as high as 15A. For a rule of thumb divide the wattage by the voltage and multiply by 2.5. Then you will never be disappointed. 

It is the continuous or 10min peak amperage you should be looking for. 

The output of a lithium battery is largely determined by the BMS (battery management system), most modern lithium batteries can put out massive currents (amps), so it is the BMS  (the small computer that controls the charging and discharging (output)) from the batteries that is the controlling factor here. 

Sealed lead acid batteries do not require BMS. 

Thorpy's Guide to Buying an EBike Battery - Where to Buy

Summary: Anywhere you like just make sure cells are branded. 

I have not included non branded cells in my search. If you are going to spend a lot of money on a battery then buy something good, that will last (hopefully).

BMS Battery - Sell some batteries made with branded cells, but others are not branded.

Green Bike Kit - Sell some good quality batteries (Headway - £300 for 48V 10AH plus shipping).

AliExpress - Search for branded cells only.  So this page shows a seller who has some samsung celled batteries for sale (£300 for 48V 20AH). Just be sure to read the description carefully.

Eclipse Bikes - Sell good quality batteries based on headway (£500 48v 15AH) & samsung (£300 for 11AH) cells.  The headway batteries they cells can be repaired and have cells replaced easily. Eclipse bike also offer a repair service.

Thorpy's Guide to Buying an EBike Battery - DIY or Bodge Up

Summary: Make Do and Mend for Savings and Satisfaction 

If you do not like spending money and have spare time do some research. You can build your own battery! Or you may be able to repair the one you have. For example a poor quality battery may be filled with 18650 cells which you may be able to replace. You could re-use the case or the BMS at the very least. 

If in my case you are swapping from a 24V to 48V system, can you use a step - up - transformer. To change 24v to 48v. Although you will need a transformer capable of handling high amperages, and a high amperage battery to account for step down in current. I may try this as this costs £18 insted of £300!

Thorpy's Guide to Buying an EBike Battery - Physical Size

Summary: Make sure it fits!

All bikes have limited space where a battery can be placed. For typical bike you are limited to a "bottle" type battery or one that fit on the rack at the rear of the bike.

Just be sure to check the battery you are getting for it size. Measure you bike to make sure it will fit, and that you have the correct mounting points.