Need help holding a 7.2V Li-Ion battery at 7.3V, need to clip 1.1V off ChargeV - Page 1
Forum section: electronics / general

author: Mr 12
author_rank: Contributor
timestamp: January 15, 2016, 04:36:23 am
content: I have a vintage detector that was designed to run on a 6V none regulated SLA battery system. The detector needs 1 AMP at start up and runs on 750-800 mah.I want to run this detector on a Sony Cam 7.2V Li-Ion battery so I am in need of a way to keep the voltage at 7.3 which is just below the 7.5V limit safety.As I understand it the 7.2V can reach 8.4V fresh off the charger so I need to clip a little voltage off until the battery is discharged about 10%.I considered using diodes but not sure this is the best way to regulate and not waste my battery.I would really appreciate any help and maybe a circuit diagram as I know just enough to get into trouble with this project.Thanks

author: tom66
author_rank: Super Contributor
timestamp: January 15, 2016, 08:32:57 am
content: You shouldn't "float charge" lithium ion cells. A dedicated charger is required.

author: tszaboo
author_rank: Super Contributor
timestamp: January 15, 2016, 09:05:16 am
content: 3 cell lead acid can be most cases replaced with 2 cell LiFePO4. The voltage is about 6.3 vs 6.6 volt. Use OV, UV cut-off, balancing the cells is not neccessary most cases, and quite difficult.

author: Mr 12
author_rank: Contributor
timestamp: January 15, 2016, 10:40:28 pm
content: NANDBlog I am using the 2 cell LiFePO4 now but want to take advantage of the Sony NP-960 battery design.With the Sony NP-960 battery design I can use the Sony charger and low battery in built protection.There is a hidden agenda here as pushing the voltage up to 7.3V helps with better stability and more depth on this machine.I think a 7.5V Zener diode could work as a voltage regulator but not sure about what happens when voltage drops to 80% due to the 0.7 volts required for the Zener to properly function.I thought this would be a simple thing for EEVBLOG members to point me in a good direction?Thanks

author: georges80
author_rank: Frequent Contributor
timestamp: January 16, 2016, 02:52:34 am
content: Brute force would be an adjustable LDO linear regulator. Burn off the extra voltage as heat, put a decent thermal path on it and you're done.cheers,george.

author: IanB
author_rank: Super Contributor
timestamp: January 16, 2016, 03:08:36 am
content: Quote from: Mr 12 on January 15, 2016, 10:40:28 pmIt's not that simple, unfortunately. The battery will start out at 8.4 V when fully charged (but as soon as you apply a load it will drop below that). The battery will go down to about 6 V at the end of discharge. So the voltage starts above what you want and finishes below what you want.If you sacrifice some battery capacity, a low drop-out voltage regulator set to 7.3 V will keep the voltage at 7.3 V or below and is the simplest and most basic solution.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 16, 2016, 04:34:46 am
content: Quote from: tom66 on January 15, 2016, 08:32:57 amIt's OK if you keep the voltage below 4.1V/cell or so (basically below the open circuit voltage after being fully charged and then left standing for a while) and limit the maximum charge current to well within limits.

author: Mr 12
author_rank: Contributor
timestamp: January 16, 2016, 05:13:53 am
content: IanB,A low drop-out voltage regulator set to 7.3 V would work, do you have a part number that you recommend and maybe a circuit diagram?If you know, will a LDO regulator use power when the battery voltage is below the regulated voltage?Thank you to all that have posted as I really do appreciate your time and thoughts!Thank you to all,