Converting a battery series connection to a parallel connection  - Page 1
Forum section: electronics / projects

author: ReadyEdi
author_rank: Newbie
timestamp: November 27, 2017, 06:59:48 pm
content: Hello,So in my current project I need to take 2 hobby high current batteries that are physically connected in series and convert that connection to a parallel connection (While still using the serial one uninterupted)between the two batteries now I just have access to the middle part of them and the + and - .Is it possible to to do that without using to many components or in terms of currents?I need to get around 10A out of that.Thanks for helping.

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2017, 07:06:56 pm
content: *NO* its not possible at all.Your only option to power a high current load at half the total battery pack voltage without a separate battery for it would be an appropriately rated Step-Down (Buck) DC-DC converter.

author: ReadyEdi
author_rank: Newbie
timestamp: November 29, 2017, 05:53:33 pm
content: You sure there is absolutely no way?How would you get high current from two batteries in series as the voltage of only one and draining both of them as if they were connected in parallel??

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2017, 06:24:13 pm
content: I've already given my recommendation. Anything else at best wont work but is most likely to start a battery fire. e.g. If you tap the connection between the batteries for the high current load, the bottom battery will discharge first, then the continued load on the two in series will polarity reverse it, killing it stone dead. Try to charge it afterwards and you'd better do it outside on concrete, wearing a Nomex fire resistant suit over heavy leathers, welder's gloves and a hard hat with a full face visor.

author: wraper
author_rank: Unknown
timestamp: November 29, 2017, 06:48:34 pm
content: Quote from: Ian.M on November 27, 2017, 07:06:56 pmYou'll destroy the battery by doing this. You cannot discharge half of the battery without causing charge imbalance which will very soon destroy the battery. As half of it will remain undercharged while another half will be overcharged.

author: Ian.M
author_rank: Super Contributor
timestamp: November 29, 2017, 07:23:09 pm
content: Quote from: wraper on November 29, 2017, 06:48:34 pmNo you wont. The DC-DC converter is there to halve the full series battery voltage, drawing on both batteries equally to power the load at a similar voltage to a single battery.Quote from: wraper on November 29, 2017, 06:48:34 pmThat's what I just said!Quote from: Ian.M on November 29, 2017, 06:24:13 pmIts a bad idea to put *ANY* load on a tap part way up a series string of rechargeable batteries. Even as low as 1% of the total load current drawn from a tap can cause enough imbalance to damage the bottom battery in repeated cyclic use.

author: wraper
author_rank: Unknown
timestamp: November 29, 2017, 07:30:41 pm
content: Quote from: Ian.M on November 29, 2017, 07:23:09 pmI read that wrong. For some reason I thought you were suggesting connecting in the middle or using DC/DC.

author: David Hess
author_rank: Super Contributor
timestamp: November 30, 2017, 07:52:25 pm
content: A DC to DC converter or inverter would easily allow halving the output voltage while doubling the current. Actually, it is possible to make a high current charge pump using big aluminum electrolytic capacitors to do this although 10 amps is pushing it. I would go with an inverter because it is the simplest.I know someone who did this for their RC car to implement "turbo". They used some high current relays to switch a pair of batteries between series (turbo speed) and parallel (normal speed) outputs but of course they could not have both at once.