Hello all. This may be a noob post but hear it goes. I'm planning on building an pedal assist electric bike so I can go on riding with my friend who has an electric bike. His bike is 52 volts with a 19000mah capacity. I would like to have similar battery capacity as him for long rides but on a budget.
I was planning on making my own battery pack using 26 inr18650-25r batteries, 13 in series and 2 in parallel for 48volts and 5000mah. Which is good for 1,920 continuous watt discharge if I'm not mistaken.
Obviously 5000mah is not gonna cut it so I was thinking of using a cheap battery with a low discharge rate but high amp capacity(If one exists) to slowly charge my battery pack over time while riding.
If I'm not mistaken you can connect a battery with the same voltage to my diy battery pack and it will charge the battery pack until they are both the same charge level. Also, does anyone happen to know if I can use a step up voltage regulator to aid with charging in order to use different voltage batteries or to allow me to fully charge the battery pack?
Does any of this sound feasible at all?
Lastly, does anyone know of any cheap high capacity batteries suited for this?
There are some flaws in what you propose.
If you charge 1 battery from another then you are wasting some of your precious stored energy as there is no way to transfer charge from one battery to another without some losses. In any case, while your primary battery is supplying the load you can't charge it as well, either it is charging or it is discharging, it can't be doing both. Your backup battery would just be supplying some of the load, not charging your primary battery, although the net effect is the same. What you are proposing is really splitting one big battery into 2 smaller ones and having one charge the other, wasting charge in the process.
Given that you are better off just having a battery with a higher capacity. A bigger battery will have an overall higher capacity than two batteries with one trying to charge the other and you will save on the complication of a charger that charges one battery from another.
I own an ebike and am active on a forum where many members upgrade. This battery is a pretty good example of what is in use on most of the upgraded bikes.
A bit over $500
edit: Oh, and +1 to the comments that using one battery to charge another battery is a bad idea. Much better to add more cells in parallel to your primary pack.
vinceherman:
edit: Oh, and +1 to the comments that using one battery to charge another battery is a bad idea. Much better to add more cells in parallel to your primary pack.
If it was a good and practical idea you would have expected someone to have done it by now.
srnet:
Lead acid car type batteries are inexpensive, wide range of capacities.
I think lead acid batteries are great for backup for things that are stationary, where space is not at a premium and weight is not a problem, I don't think they are much use on a bike....
PerryBebbington:
I think lead acid batteries are great for backup for things that are stationary, where space is not at a premium and weight is not a problem, I don't think they are much use on a bike....
To heavy for the bike sure.
You would put them on a little trolley and tow it behind, like one of those childs trolleys you can get.
Thanks everyone for your replies. I ordered 52 of the 18650 batteries. should provide 10AH for now, I can add more batteries later if needed. Now I just need to find a fat bike suited for this, easier said than done with the current market.
05silgto:
I can add more batteries later if needed.
Errrr.... sorry but it is never a good idea to mix batteries of different ages, they will have different characteristics as ageing changes them, in particular they lose capacity as they age. I would only ever combine multiple cells together when they are all new and ideally from the same batch, or at least from the same manufacturer.
mAh is a measure of capacity, not maximum discharge current.
mAh or Ah is capacity (amp hours - 1 Ah is 1 amp for 1 hour) but this spec does not say anything about discharge rate . You could have two 2500mAh ~3.7v batteries, and one could be rated for 500mA tops, while the other is meant for a vape and could push out 20A.
The main problem with your idea though is that there isn't much to gain by trading off discharge speed for cost/capacity if you care about weight. There is no competing battery type that maintains the same energy density (or anything close to it) for a lower price. Certainly nothing that would make up for the inefficiency of charging one battery from another
srnet:
If it was a good and practical idea you would have expected someone to have done it by now.
Charging one battery from another? Yes TONS of people do this, and you can buy them at any electronics store. Those power banks to charge your phone from - what do you think they are? They're a battery, that you use to charge another battery (the one in your cell phone) from.
However, this isn't relevant for OP's problem - they make sense because the battery in the phone is forced to make certain tradeoffs in terms of size and weight that severely limit it's capacity, while something you carry in your backpack or purse can be larger with the same energy density (or worse), and because most phones don't even let you swap the batteries anymore.
Hi,
If you are going to make your own battery pack, you will need to consider a Battery Management System to ensure even charging of all those batteries in series.
Your friends bike, if it is a commercially available battery will have a BMS built in as a proper monitoring system.
That sounds so problematical. If you need occasional extra power boost, just switch over from one battery set
to the other, then back again. Simple, but you need two "fuel gauges"...
The low current battery is probably going to have its hands full just propelling the bike, if it has power to
spare to charge another one, its got power to spare for hills and sprints.
You could be smart with the voltages and use diodes so that when the low current battery is struggling the
high current one automatically contributes, but its tricky to get this working nicely over time and temperature
and cell discharge history(*) - better to simply switch over I think.
(*) internal resistance and open-circuit voltage in cells depends on its temperature, the age of cell,
current state of charge and probably a few other things...
