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Looking for some help with correct wiring: I have created a custom 7-cell battery (4 cell block in parallel, 3 cell block in parallel and both blocks connected via series) to produce a 7.4V 23.8A battery set/unit. At a basic level, I need to connect the battery directly to an LED array (parallel) which requires 9V @ 800mA or 9V @ 1.6A (constant current). Since the battery has an output of only 7.4V I need a booster, I know. I also know that I can get a DROK CC booster from Amazon that is configurable. However, I do not have the space to house these larger prefabricated PCBs. I need only the critical components to squeeze into a 2" space and get a working proto. Could someone please provide the correct parts to purchase from Digikey or Mouser (or where else) and make this work? I am also open to any other suggestions in configuring the battery.

Then there is charging! I wanted to use a CC power adapter that would pump 7.5V at 1.6A. Will this charge the complete 7 cell battery unit or would I need to reduce that to the each cell level of 4.2V @ 1C? Appreciate any feedback. Thanks

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    \$\begingroup\$ You MUST NOT!! connect a group of four parallel-connected cells in series with a group of three parallel-connected cells - the four-cell group will be under-charged while the three-cell group will be over-charged. \$\endgroup\$ – Peter Bennett Dec 12 '18 at 22:49
  • \$\begingroup\$ Thank you Peter. Then the only alternative would be to keep them all in Parallel seeing how a large capacity is very important... This means, we have to boost the voltage and run a CC not to exceed 1.6A. Right ?? \$\endgroup\$ – Alex Dec 12 '18 at 22:59
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    \$\begingroup\$ No. You can connect a 3P in series with a 3P, or a 4P in series with a 4P (if you can find room for an eighth cell). \$\endgroup\$ – WhatRoughBeast Dec 12 '18 at 23:04
  • \$\begingroup\$ Ah, I see what you are trying to say - staying even in the cell numbers !! If I can get 3500mA cells, then I can maintain the 20Ah capacity I am looking for and configure a combo Parallel/Series battery pack. \$\endgroup\$ – Alex Dec 12 '18 at 23:10
  • \$\begingroup\$ Balanced cell charge is the key to achieve full 500 cycles. Whenever they become unbalanced, the weakest cell, will die faster than the rest. Conservative use ( 4.0 max 3.0V min) and balanced charge/discharge is very important if you want reliable use, so design for these cutoffs. Otherwise if you dont care use 4.2, \$\endgroup\$ – Sunnyskyguy EE75 Dec 12 '18 at 23:39
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Do not do this! Lithium ion batteries are prone to catching fire (the technical term is "vent with flame"), and need to be assembled into a proper battery pack with a protection circuit to be safe. And you always need the same number of parallel cells in a series-connected string.

As a first step, I recommend you do some extensive reading on lithium-ion batteries, chargers, battery management systems (BMS) and protection circuits, and why they are needed. Battery University is a good place to start; this is their page on Making Lithium Ion Safe.

Once you have a solid understanding of Li-ion safety, assuming you still want to use 7 cells, I would recommend using protected cells (with built-in safety cutoffs) and connecting them all in series, then using a buck converter to get your needed voltage. For charging, you can either remove them from the device and charge them in standard single-cell chargers, or bring taps between all cells to a connector and use an off-the-shelf 7-cell balancing charger.

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  • \$\begingroup\$ Thanks Nate. Appreciate the guidance. The 7-Cells are the Protected version and they were going to be configured in a pack using tabs (for sure). Since battery capacity is more important in the limited space of my prototype, I have to use in Parallel configuration with a booster (I assume) as the only option. Ultimately, a new PCB has to be designed to fit this space with the proper BMS and temperature controls for safety. As for charging, I was thinking of using the Wall charger with CC (max 1.6A). Am I on the right track a bit ?? \$\endgroup\$ – Alex Dec 12 '18 at 23:05
  • \$\begingroup\$ 7 cells will provide the same Wh capacity regardless of if you use them in serial or parallel -- the buck converter will reduce the current drawn. The problem with protected cells in parallel is that when the first one goes open circuit, the current draw from the rest increases, which is not ideal. As for charging, is the wall charger you're thinking of a proper lithium-ion charger, or just a CC power supply? \$\endgroup\$ – Nate Strickland Dec 12 '18 at 23:14
  • \$\begingroup\$ The charger is a standard USB charger that has variable output - so I can set to 7.5V and since it has a max current of 2A, it will keep drawing until its cap is reached - in which case the LED will only draw what it needs 1.7A. This is a standard CV charger. If I purchase the CC charger, then I can setup a cap on the current and it will reach it at the Voltage of around 9.6V and keep it constant there. I guess getting non-protected batteries will then require the customized circuitry that I would ultimately need. Thanks again \$\endgroup\$ – Alex Dec 13 '18 at 2:00
  • \$\begingroup\$ Ideally you need a charger that can do both constant current and constant voltage modes, although you can get away with CC-only if you don't mind sacrificing some capacity (your cells will actually last longer this way). \$\endgroup\$ – Nate Strickland Dec 13 '18 at 2:38
  • \$\begingroup\$ More info on proper Li-ion charging techniques: batteryuniversity.com/learn/article/… \$\endgroup\$ – Nate Strickland Dec 13 '18 at 2:38

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