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I want to build a system powered by 14.8v Li-po battery pack or mains power (selectable power path). When the mains adapter is connected, the battery will be charged and the system will be powered from the adapter.

I've found a family of standalone charger chips by Texas Instruments doing exactly that. It's the bq246xx. This family has some features like over/under-voltage/current protection.

Since the battery I'm using is a 14.8v 4s Li-po battery I must use a cell balance chip. I found this one bq76920 by Texas In. Balancer chips also contain similar features to the charger chips (o/u-v/c protection).

So my question is: Do I have to use both a battery charger and balance/monitor chips to charge and balance the battery and power the rest of the system at the same time, or can I use only the balancer chip to do these things.

My battery pack is quite expensive ($120) and other people will be using it, so I need a solution that offers maximum safety for the batteries & users.

Thank you for your time

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  • \$\begingroup\$ yes i just meant 4 batteries in series pack. The 4s1p is usually used in RC forums so i got used to it. \$\endgroup\$ – Sparky Feb 1 '16 at 11:17
  • \$\begingroup\$ comment moved to answer. \$\endgroup\$ – Robherc KV5ROB Feb 1 '16 at 14:23
  • \$\begingroup\$ FYI: I checked on that 2nd chip you listed, and it's a monitor-only chip. It can monitor health of the cells, but would need external circuitry to handle all charging/balancing. \$\endgroup\$ – Robherc KV5ROB Feb 1 '16 at 16:05
  • \$\begingroup\$ @RobhercKV5ROB true, you would need both chips: a bq246xx family chip (or similar) for charging and a bq76920 (or similar) for balancing. The bq76920 does have balancing: it can shunt 70 mA accross any of the cells, dicharging cells selectively. Oddly enough complete BMS chips that include both the DC/DC converter and the balancing circuitry seem to be rare. \$\endgroup\$ – jms Feb 1 '16 at 16:52
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Since you stated that you want optimal protection for your batteries, I think you'll like this solution.

I came up with a circuit that takes 4 T.I. BQ24070 charge controllers (with integrated charger/battery supply switching) as the only battery ICs. This circuit allows each battery to be independently charged & discharged, with all (functional) batteries working in balanced parallel to power your load (through a Boost converter). From the information you gave, I think you'll like that each cell is balanced & protected (from both overcharging & undercharging), and the device has fail-over capability in case of one or more batteries' failure (can supply full output voltage from 1-4 batteries).
Independent Batteries

  • The non-labelled resistor should be sized as appropriate for the NPN transistor whose base it feeds.
  • The zener diode & 10K resistor will push 0.7V to the NPN transistor's base, pulling the AO3415 MOSFET's gate low, and enabling output from the cell as long as cell voltage remains above 2.7V (undervoltage/overdischarge protection).
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  • \$\begingroup\$ Prev. answer deleted & new answer posted to answer OP's additional criteria, without carrying-forward now irrelevant (to thks question) comments. \$\endgroup\$ – Robherc KV5ROB Feb 5 '16 at 15:35
  • \$\begingroup\$ This answer looks good but have you actually tested this circuit? \$\endgroup\$ – Sparky Feb 6 '16 at 9:08
  • \$\begingroup\$ Also, the battery I am using is a 4 series pack with 2 fat cables for power and 5 thin cables for balancing. So, from the circuit above i guess that I will be using only the balancing cables to supply the battery pack and not the actual supply leads ( the fat ones). Is that correct? \$\endgroup\$ – Sparky Feb 6 '16 at 9:39
  • \$\begingroup\$ No, it would be impossible for me to test this circuit without sourcing the parts, along with 4 LiPO cells, and building it. Since I have no current need for a 4-cell, ultra-reliable LiPO pack, that's more $$ than I'm willing to spend with nothing gained. As for the pack-included cables, that is inflrmation which would have been very useful to give us before I spent several hours researching parts & building that circuit diagram. This circuit would necessarily use only the "5 thin cables" for both charging and discharging the pack. \$\endgroup\$ – Robherc KV5ROB Feb 6 '16 at 11:57
  • \$\begingroup\$ If those cables can handle the current you're needing (this circuit is capable of up to 2A charge/discharge current per cell), then that may still be a perfectly usable answer for you, but if you need higher output current than you can trust those wires for, you would need to eliminate everything to the right of the cells in this circuit, add your own batt/charger source selector, and add overdischargs protection to the "fat cables" to use them for the series output. \$\endgroup\$ – Robherc KV5ROB Feb 6 '16 at 12:03

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