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# 18650 4.2V to 20V

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#### SentinelAeon

##### Member
Hello,

I am trying to simplify 1 of my speakers. Right now its 5S running a 20-25V amp. I am trying to now replace it with batteries in parallel and i need step-up that will convert that 4.2V to 20V. Usualy i find everything on ebay for decent price but now i cant find a nice step-up.

I need step-up like this:

- input voltage 2.8V
- output voltage 20V
-allowed amperage = 10A

edit: if i dont find the right step-up, could i just use 2 step-ups of 5V instead ? I will charge all batteries with 1 charging usb board and i dont have trouble connecting to all the batteries. And i split the batteries into 2 groups, 1 group goes into each step-up. That way i only need 5A step-ups.

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Would this work with 2 step-ups ?

You need 20 to 25V at 5A? That is 100 to 125 watts. That will require 23 to 30 amps from your poor little batteries.
"batteries in parallel" How many? Assume 2, you could series and get 8.4V which will boost better. (need 11 to 15A)

Why do you think you need 10A?

5S is 21V when fully charged and is 16V when the load should be disconnected.
21V x 25A is 525W. Isn't the current at 21V actually only 2A to 2.5A? Then 5 cells in parallel must supply a total current of 12.6A including heating of the voltage converter.

I can't find your thread about the amplifier and how it is powered. What will you use to charge the 5 cells in parallel with 5A? Charge at only 1A for 2 days and 2 nights?

My bad, input will be 12.6A yes, output will only be 2.5A since i am powering a 50W amp. I want to avoid putting batteries in series, so i want step-up that can take as low as 2.8V as input voltage, it must handle 12.6A input and 2.5A output.

As for charging, i will charge with 1 or 2A yes, and it will take a while but its not a problem, i charge it usualy over night as for powerbank charging, it will spread through all batteries so it will be ok

Now you show two battery cells in parallel. The charger might overcharge one cell (kaBOOM!) and undercharge the second cell, because a battery is charged with current, not voltage.
The voltage step up circuits might not like their outputs to be in parallel.

You can charge two (or more) cells in parallel, but you do have to make sure that the cells have the same characteristics and the wiring layout is such that all the cells have similar resistances. Also you must make sure that the cells are in the same state of charge, and the same voltage, when you connect the cells together.

Large battery installations have lots of cells in parallel. For instance a Jaguar iPace has groups of 4 cells in parallel, and 108 of those in series, totalling 70 kWh.

I agree that the step-up circuits might not work well in parallel.

Well then the solution is only 1 step-up but i have to find the right one.

I trust my charger to charge the batteries in parallel, i plan to use at least 5 of them. And i trust that they will balance themselves

i plan to use at least 5 of them
I know series charging is a pain but; 5 batteries in series gives you enough voltage to run the amps with out a boost (step up) supply.

Well then the solution is only 1 step-up but i have to find the right one.

I trust my charger to charge the batteries in parallel, i plan to use at least 5 of them. And i trust that they will balance themselves
Are you going to disconnect the battery cells from the series connection for charging? How? Aren't the battery cells soldered together in series?

Are you going to disconnect the battery cells from the series connection for charging? How? Aren't the battery cells soldered together in series?

Li-Ion batteries work, and charge, perfectly well in parallel, and it's EXTREMELY commonplace, as they do in series - although it's a good idea to include a charge balancing board.

However, I fail to see any reason for putting his cells in parallel rather than in series?.

Well it is really simple. There are 2 reasons, keep in mind that i am doing a budget thing so every dollar or euro reduction in cost matters. Now there are 2 reasons:

1) i use a cheap 5S BMS right now. It does the job well, disconects the cells when they are to low, etc. It costs 1.5 dollars which is great. The problem is, it doesn't have balancing. And what happened is that cells got out of balance. So now when i charge them, some of the cells get to 4.1-4.2V fast and stop the charging process, while other cells are still low. And when i use the speaker the less charged cells get to this 2.8V point and turn the speaker off even though there is still juice in other cells. Speaker plays less time then it did when i assembled the device.

2) it is absolutely necesary for the speaker to have micro-usb charging port. Why ? Very simple, sometimes it has to play longer then i thought it will and i always carry a power bank with me. And i simply want to be able to charge it like that. Also sometimes i go to other places where they dont have 21V charger, but everyone has a micro-usb charger. This saves a lot of trouble and makes things a lot easier.

Now if i could solve the balancing thing and micro-usb charging some other way, i wouldnt have trouble with staying in 5S. But series + step-up solves both those problems. The only problem is that as far as i know, it is impossible to find a cheap step-up that will do what i need it to do.

Is there any reason you can't add battery balancing or change the BMS for one that includes it?

Also, if your amplifier takes 50 W, and you want to charge the batteries from a power bank, the power bank won't give you many minutes of running, and it will take hours to get the energy from the power bank, via USB, into the batteries.

Well the reason was that i couldnt find any cheap 5S BMS that includes balancing. I only found separate cheap balancing board. Would something like this do the trick:

If i add it to existing setup ?

Well the reason was that i couldnt find any cheap 5S BMS that includes balancing. I only found separate cheap balancing board. Would something like this do the trick:

If i add it to existing setup ?
That would probably work. It's quite simple, and seems to be just device that puts a 66 mA load when the voltage gets to 4.2 V. It would allow slow charging to continue when some cells are fully charged.

You don't need a the balancing to work quickly. It only has to keep up with leakage in the cells, which is minimal.

Balancing during series charging is very important when lithium cells do not match. Old cells usually do not match. Without balancing then a good cell becomes fully charged but the weaker cells are still charging which causes the good cell voltage to become dangerously too high.

The extremely cheap and very simple 66mA "balancing circuit" might be used for cheap little Li-PO battery cells that charge with and provide low currents.
Other people have posted links to ebay 5S balancing circuits designed for 18650 powerful Lithium cells.

Hmm, i already ordered those balancing boards. So they will not do their job ?

The sales sheet does not say who makes the cheap simple low current balancing board and does not recommend a maximum battery capacity for it.
It could be used with new name-brand 18650 battery cells. But didn't you say your cells are old and are different brands?

Hmm, i already ordered those balancing boards. So they will not do their job ?
They should be fine, as long as you also have the protection board connected.

That will shut off power any time a cell reaches maximum voltage, then the balance board can catch up with the equalisation if needed.
The first full charge may take a long time if the cells are badly mismatched.

Just try it. As long as the protection board remains in circuit no cells can go beyond their safe range, and over a few hours you should see the lower voltage cells gradually come up to match the highest ones.

This overlaps a bit with what rjenkinsgb has just said, but I started writing it before he posted.

An 18650 cell can be 3000 mAh or more. The 66 mA would balance a completely imbalanced set in around 50 hours.

However, cells going out of balance is something that takes time as well, so if you balance a series set first, a small balancing circuit is probably all that's needed. Really good quality batteries have balancing systems, and they are only capable of very slow balancing rates. All the balancing circuits that I have seen are resistor banks within the battery, and they are not sized to get rid of a lot of power. However, the cells are guaranteed to be very similar, and the battery management circuit can detect an imbalance and turn on the balancing at any state of charge.

On the simple circuit you have, the balancing will only happen during the charging, and then only when one cell gets fully charged.

The balancing does rely on the charging system limiting the battery voltage near the voltage of the balancing circuit. If the charging circuit cuts out as soon as a cell reaches 4.2 V, the balancing won't really work.

Say you have two cells in series, and one is at 3.9 V and one at 3.7 V. Fast charge will result in the voltage rising quite quickly, and soon the cells will be at 4.2 V and 4 V. At that point the balancing board will start to bypass 66 mA from the cell at 4.2 V. If the charging circuit holds the largest cell voltage at 4.2 V, then the lower voltage cell will be charged at 66 mA more than the high voltage cell, and eventually the lower voltage cell will rise to 4.2 V as well, and balancing is complete.

However, if the charging circuit keeps supplying a high current, much more than 66 mA, until the battery is at 2 * 4.2 V, the higher voltage cell will be overcharged, and may be damaged before the total voltage is 8.4 V which will be when the current drops. When that happens, the 66 mA will eventually bring the higher voltage battery down, but it's not a good way to treat the cells.

Also, if your charger prevents the cells from ever getting to 4.2 V, or cuts out completely as soon as one does, then there will never be any time when the balancing is working,

So, as usual, the devil is in the detail. That was why I said before that the balancing circuit "would probably work". If you do use it, monitor it carefully during a charge cycle and make sure that it is doing what you want. Write down times and all the cell voltages and prepare for a boring couple of hours.

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