battery Custom made battery bank

[Wolf585]

1. I was planning on making a large battery bank from 20 18650 batteries, and since that might take a while, I was wondering if I would be able to attatch multiple charging inputs for faster charging times. Or is there a limited input speed. Please tell me if there is a better way to do this than shown in the diagram

Thanks. 

[Windows7ge]

Their's definitely a limit. Charging too quickly could be not just a fire-hazard but decrease the lifespan of the cells.

[Mira Yurizaki]

You can connect multiple power supplies in parallel for higher current, but you better make sure where they end up going can handle it: http://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=520808&nid=-11143.0.00&id=520808

 

As for a limit in "input speed", yes. There are limits to how much current you should pass through to charge. High current can heat up the batteries. It can also cause them to wear out faster (probably due to said heat buildup).

 

If you want to avoid this, you're going to have to find a way to isolate the batteries into groups and charge them independently.

[mariushm]

You can't do it like that.

 

Lithium batteries can't be charged directly by pushing some voltage in them, you need to use a circuit that varies the voltage between around 3.7v and around 4.25v while limiting the current to a reasonable amount (let's say 500mA to 1A for 18650 batteries) so that they won't overheat (the cooler they are and the slower the charge current, the more charge cycles the batteries will have without degrading to a lower total capacity).

 

Smart chargers have multiple "channels" and each channel charges a group of batteries separately. at the same time, they constantly monitor the voltage on each battery group so that the voltage on each group will be very close to the other battery groups.

If the voltage of a battery group is higher than another, the smart charger connects a load for a brief amount of time (a resistor for example) to consume some of the energy in that group of batteries and get the voltage more in line with the other batteries.

Otherwise, the higher voltage batteries may push a lot of power into the group of batteries with lower voltage in order for the voltage of all groups to be evened out and that high current can weaken the batteries.

 

What you can do is create groups of batteries wired in parallel  (to have same 3.7v ... 4.2v but higher capacity ) and you can use a separate charger for each group of batteries. When you're done charging each group of batteries with their own charger, you can connect the wires of each group together, either in series to get higher voltage or in parallel to get more capacitance.

 

You can create very cheap battery chargers with ICs like MCP73811 which are stupid easy to use, they take in up to 6v and charge whatever you connect to them at up to 450mA. So if you have groups of 5 batteries in parallel, each will charge at around 90-100mA.

Anyway, the IC above is cheap and sort of dumb, in the sense that it won't disconnect itself from the battery when it's done charging it , which for a basic charger is not big deal.

 

[the dutch guy]

get one of these:

http://www.ebay.com/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Supply-Module-7-32V-to-0-8-28V-12A-RF-/172386544132?hash=item282309ea04:g:p2wAAOSwo4pYDdqb

 

hook it up to a old PSU you have lying around (connect green wire to black to hotwire the PSU) and set the dc dc board with the trimpot to 4.05v (not 4.2, that will just kill the battery faster. with 4.05 you lose about 5~7% battery capacity).

crank the amps up to 10A (short out the output and measure with the high current inpot of your multimeter. do mind you do not load the multimeter no longer then 20 seconds and let it cool. otherwise you will burn out the multimeter. a fan is mandatory for this converter if you run it at high currents.

 

you can charge such a large battery it in a few hours like this. the poster above me is not thinking at the right scale here., charging with the mentioned IC will take at least 5 days. my solution costs 5 bucks and charges it in less then 5 hours wich is very slow for lipo standards.

[Enderman]

You're putting batteries in series without a BMS, so you need a balancing charger.

You can't just plug it directly into a power supply.

[Wolf585]

On 5/20/2017 at 0:31 AM, Enderman said:

You're putting batteries in series without a BMS, so you need a balancing charger.

You can't just plug it directly into a power supply.

ah, yes sorry. I was extremely stupid to this subject last month and am frankly ashamed of how dumb that idea was.