Adding reverse polarity protection help - Page 1
Forum section: electronics / general

author: bigjoncoop
author_rank: Regular Contributor
timestamp: December 15, 2023, 01:44:38 am
content: Ive got a couple of RC lithium battery chargers and would like to add reverse polarity protection.The chargers are ISDT Q6 Nano's. Now the more expensive Q6 plus's have fast blow fuses on the output of the Chargers.The ones I have are a slightly cheaper version and they are using PTC resettable fuses instead of fast blow fuses... but they do not seem to protect the output mosfets from reverse polarity since I have recently blown two of these Chargers up due to reverse polarity.On both of them one of the output mosfets blew up...So I'm wondering if replacing the PTC resettable fuses with normal fast blow fuses might do the trick?Or if there is a simple method on implementing reverse polarity protection on the output of the charger that will not interfere with the charger functions. ANY IDEA'S?THANKS

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 15, 2023, 01:57:30 am
content: Not aware of any simple electronic fixes. In general there are no fuses that are fast enough to protect FETs.The usual approach to this is polarized connectors. Several, but not all of the connectors used in RC are polarized. Color coded wires and painted cues on non- polarized connectors can help, but Murphy always gets by these precautions. Two prior failures indicates that you are one who needs more robust protectionThere is one case where there is a simple fix. If your problem is at the input of the charger (clipping to a car battery for example) you can put a full wave bridge rectifier in the lines leading to the charger. It will prevent connect the charger jmproperly regardless of how you connect the input leads and will have little or no impact on charger operation. Be sure to select a bridge capable of handling the current you expect to draw

author: perdrix
author_rank: Frequent Contributor
timestamp: December 15, 2023, 02:30:13 am
content: I don't know if it works for this circuit, but take a look at this:If connected the right way round the MOSFET is just fully on, if the wrong way, the gate is pulled -ve which shuts everything off. I used an IRFS7537TRLPBF FET.David

author: thm_w
author_rank: Super Contributor
timestamp: December 15, 2023, 10:35:10 pm
content: https://www.eevblog.com/forum/chat/replacing-fast-blow-fuse-with-ptc-resettable/

author: nctnico
author_rank: Super Contributor
timestamp: December 16, 2023, 12:23:29 am
content: Put a (fat) diode across the mosfets which conducts when the polarity is reversed. Make sure there is a fuse in the path to the batteries though.

author: AndersJ
author_rank: Frequent Contributor
timestamp: December 16, 2023, 01:45:12 am
content: Quote from: nctnico on December 16, 2023, 12:23:29 amThat would do the job,IF the battery is charged when connected the wrong way.But the challenge remains, if the batteries are empty when connected.

author: AVGresponding
author_rank: Super Contributor
timestamp: December 16, 2023, 10:36:42 am
content: Quote from: AndersJ on December 16, 2023, 01:45:12 amIt's impossible to make things completely idiot-proof; nature invariably produces a better idiot. At some point you just have to accept the inevitable and say "check the f#!king polarityBEFOREyou connect it!"

author: nctnico
author_rank: Super Contributor
timestamp: December 16, 2023, 04:20:05 pm
content: Quote from: AndersJ on December 16, 2023, 01:45:12 amI'd expect a decent battery pack to have reverse charging protection. With the diode anti-parallel to the charger, at least the charger is protected.But having connectors which can't be plugged in wrong, is a first step to take.

author: macboy
author_rank: Super Contributor
timestamp: December 17, 2023, 03:01:25 am
content: Quote from: nctnico on December 16, 2023, 04:20:05 pmR/C batteries are just bare LiPo cells in some series and occasionally parallel configuration, with a layer of shrink wrap for physical protection. Maybe a thin plastic "hard case" which is only a little better. No BMS or protection circuit of any kind. They are quite dangerous, many can easily supply hundreds of amps continuously. I have some with 2 milliohm ESR per cell. Do the math on short circuit current, it is scary.You are right about polarised connectors. With batteries having such high energy capability, polarised connectors are a must.

author: EPAIII
author_rank: Super Contributor
timestamp: December 19, 2023, 01:34:13 am
content: If the batteries are empty when connected?I can't say I have never seen a completely discharged battery and my experience is limited when it comes to lithium cells, BUT most batteries will have some charge remaining, even after heavy use. So, is that really a problem? Just asking.As for the original question and the challenge of making it idiot proof, the real problem seems to be that the current flows one way when using the batteries and in the opposite way when charging them. I am assuming that the battery packs are removed from the model airplane for charging. That would allow flying with one battery while charging another.SO, I believe I have seen NiCad batteries that had one connection for use and a separate connection for charging. Both were built into the same connector for a minimum parts count, but they used different pins. This suggests that a simple, three pin connector could be used. One pin for common or negative, I guess, and then a positive pin with a direct connection for use and a second, positive pin with a diode for charging.So in use current flows out the first positive pin with nothing to interfere. You get maximum power - no Voltage loss in a diode or other circuit. And when charging the second connection is used and the current can only flow one way, the correct way, with the steering diode.I looked at some RC batteries and they seem to come with a variety of connectors. Most already have two connectors and perhaps one of them could be repurposed as the charging connection with a different pin arrangement. But the charger would also need to be modified. Perhaps it would be easier to modify the connector for actual use of the battery, perhaps with a three pin, omitting the center contact.Another thought would be to epoxy the two connectors together, but staggered. The one sticking out the most would have the steering diode added and it would be used for charging. No modification to the charger would be needed. The one that is set back some distance could not be physically plugged into a connection on the charger but a cable in the airplane would easily reach it. It would have no diode and would be used for actual flying.If both of the original connectors are needed while flying, then a third connector for charging could be added, probably to one pair of wires and with the steering diode. The two original connectors would be attached on two sides of the charging connector and both would be staggered to the rear to prevent them from being plugged into the charger.Quote from: AndersJ on December 16, 2023, 01:45:12 am

author: EPAIII
author_rank: Super Contributor
timestamp: December 19, 2023, 01:41:06 am
content: A further alternate arrangement for my idea above would be to have both of the original connectors separate as they are now, but with a piece of insulation material (PC board scrap) epoxied to them, sticking out the forward end, so they can not be plugged into the charger. Then a third connector would be added with the steering diode for charging. It would be the only one that physically could connect to the charger.This means no changes to the charger and all the mods are made to the batteries. Any charger with the same connector could be used. It's the batteries that are made fool proof. And a completely dead battery connected backwards would get no charging current through the diode. Your improved version of the fool is going to have to be a lot better fool to get around that.A small cable tie wrapped around the connector and insulator finger combination while the epoxy is still curing would make that sandwich a lot stronger.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 19, 2023, 05:33:19 am
content: There is no need to overcomplicate the connector situation.Current always flows out of the battery to the load in use. Designate that lead as the red lead, and if you are thinking in terms of conventional current, current is flowing out of the positive terminal.Current always flows into the battery during charging through the same lead that supplies current to the load in use. That lead has been designated red.Now matching colors the load wire receiving current is red, and the supply wire charging the battery is also red.All these conditions are met with a simple two terminal polarized connector. Most RC plane batteries I have encountered come either with a polarized connector on the load wires, or bare wires to allow user installation of appropriate connector. The second connector seen on many (but not all) RC batteries has low current connections to each internal cell for use in balancing the charge on the cells. The wiring on this connector cannot support normal charging current.The only connectors that do not force correct orientation I have encountered in RC planes are the "Anderson Power Pole" connectors (which are polarized, but whose geometry allows a chain of connectors to result in crossed polarities) and those who do not use a unitized connector and use bullet connectors for everything. Those who choose to use the latter two connection styles must recognize that their choices require a high degree of care. The OP having had two failures should not choose to use either of these styles.Even with a polarized connector on the battery, charger and load there are opportunities for mistakes. Many modelers wire their own connectors and can make mistakes. I have seen planes and batteries for sale that use the same style of connector but with different conventions. Many chargers use color coded banana pins to connect the charging cable, and these depend on the operator to follow the color code.Most of these failure opportunities can be avoided with a one time careful inspection (and perhaps testing with a simple idiot light polarity detector) at time of purchase or manufacture, so the most dangerous of these failure modes is the banana connection to the charger. For those prone to careless mistakes it may be worthwhile to make this idiot resistant by gluing the charging cable in place, adding a mechanical interlock to prevent reverse connections (could be implemented with 3D printed parts glued to the charger and charging cables), or replacing the output banana jacks with a polarized connector.

author: EPAIII
author_rank: Super Contributor
timestamp: December 19, 2023, 06:28:45 am
content: And yet the OP has apparently seen some situations where things were connected incorrectly. Perhaps some with dangerous consequences. Just because someone flies a RC aircraft does not mean they understand electricity and the possible dangerous consequences of the misuse of lithium batteries.Some situations do call for measures that are as fool proof as possible.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 19, 2023, 09:05:15 pm
content: Quote from: EPAIII on December 19, 2023, 06:28:45 amYes, all idiot proofing fails in the presence of a superior idiot. He apparently did not use polarized connectors, or did not verify that his polarized connections were wired properlySimilar omissions would cause the failure of any potential failure correction circuitry.

author: EPAIII
author_rank: Super Contributor
timestamp: December 21, 2023, 02:20:31 am
content: I thought this thread was about how to ADD PROTECTION, not about the need or value of that protection vs. the potential stupidity of some users. I attempted to address the OP's question. Not start a debate on a different subject.I believe I posted one possible way to do what the OP asked for.