DIY Power Bank technical questions  - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 31, 2019, 09:38:47 am
content: I'd like to make a prototype power bank that could charge 4 phones at once at full speed (2A) for 8 hours a day to be used for commercial applications.There will be no fast charging circuitry to keep costs to a minimum. If my math is correct, It will require 4x2A=8A of continuous current draw x 8H =64Ah x 1.2 assuming 80% efficiency for buck/boost converter + cabling power losses so I will require 76.8‬ Ah battery bank.What would be the best way to assemble such a pack? should I parallel 4 x 20Ah Li-ion packs? parallel 22 3500 mAH 18650 cells? What combination would be most compact and efficient?What about the voltage regulation/charging circuitry?

author: angelicajames
author_rank: Newbie
timestamp: May 31, 2019, 10:12:04 am
content: Maximum of the battery chargers don't have current and voltage ordinance requirements. The step-down voltage is just utilized for charging. These chargers increase domestic revolution so the output voltage declines when the battery is united to the charger.Further, fluctuations in the AC line more influence the charging means. This current and voltage regulated charger discharges these hindrances and can provide well regulated 12 volt DC for charging.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2019, 10:38:43 am
content: You have a slight miscalculation due to mixing amps at 5V and amps at 3.6V(avg). Calculate for output power, divide by efficiency estimate and then work backwards to get the input current at average battery voltage. (It's good to calculate the input current at min & max cell voltages as well, for designing the wiring, fusing and DC/DC. For calculating Ah required, the average will do.)80% is a poor efficiency target. Aim higher, at least 90% - say 93% for the DCDC and 97% for wires, fuses, etc - you'll save it back on cells, and thermal management becomes easier, so you save on heatsinking as well. After all, 20% of 8A*5V is whopping 8 watts you don't want to dissipate on a PCB!The sane choices are either 1s with synchronous boost, or 2s with synchronous buck. Non-syncronous are way too inefficient for such high power, IMHO.In a thermally coupled, small 2s system with quality brand cells, there is absolutely no need for center tap monitoring or balancing.Remember to have a low-voltage cutoff, and make sure leakage after the disconnect doesn't kill your pack.If you go for a boost, a separate FET switch to prevent unlimited current through the boost diode (or the body diode of the synchronous switch) is highly recommended. Integrated "e-fuse" power switch ICs do exist for this purpose.Personally I would do with paralleled 18650's, but nothing fundamentally wrong with pouch cells, either. It just seems to me that still in 2019, it's easier to get good brand 18650's from gray brokers, than proper pouch cells. In li-ion cell safety, brands do matter.Remember traditional (single-use) fusing right at the battery, in case any electronic protections fail, to prevent fires.What are your requirements on the charging of power bank? This defines whether you can find an easy off-the-shelf charger IC or not.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 31, 2019, 10:53:58 am
content: Thanks for the clarification.I'll want to charge the pack entirely in less than 16 Hours with either a usb port or maybe an external 12V charger.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2019, 11:00:49 am
content: 8A * 5 V * 8 hours / 0.90 (eff) = 355Wh355Wh/16hours = 22W charging power. At 5V, 4.4A -> impossible with standard 5V USB.An external 12V charger would probably be the best. This would then be another synchronous buck. Integrated li-ion charger ICs with synchronous buck FET drivers do exist. Some can control input back-to-back FETs, which is desirable unless you can 100% guarantee mechanically that the 12V charger input is inaccessible, since unlimited battery voltage flows through the synch buck to the charger input.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 31, 2019, 07:30:06 pm
content: Quote from: Siwastaja on May 31, 2019, 11:00:49 amI have decided to make a 12V pack with a 3s6p configuration.Will I need 6 protection boards or just one since they are all in parallel?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 31, 2019, 07:58:45 pm
content: Paralleled cells act like one bigger cell. Just measure each voltage individually so they are within about 50mV before doing the initial connection, to prevent excessive current from flowing when you make the connection. Make sure the parallel connection cannot accidentally break (and later reconnect).

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 31, 2019, 08:02:14 pm
content: Yes of course i will charge them all to 4.2V prior to welding them in the configuration mentioned.So you're saying one protection board is all I need then right?

author: viperidae
author_rank: Frequent Contributor
timestamp: June 01, 2019, 07:33:14 am
content: You'll find you won't need 2A if you don't have fast charge negotiation. Most phones won't pull more than about 1.2AUSB micro is only rated at 1.8A max

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: June 01, 2019, 09:13:18 am
content: Quote from: viperidae on June 01, 2019, 07:33:14 amSure however I have to design it for worst case scenarios with large tablets in mind so 2A per port would be more than enough and leave some headroom.