I have a 64Ah EFB car battery in a 2018 Volkswagen car with start/stop technology. I have an Optimate 5 charger which supports EFB and AGM start/stop batteries:https://optimate1.com/om5ss/

I would like to understand if I need to disconnect the battery before charging it. If so, both cables or just the negative?

Some say not disconnecting it can damage the ECU, some say they have been charging without disconnecting for decades and never had any issues, some say it depends on the amperage - my charger has a 4A output.

Before I get crucified because similar questions exist, yes, I have seen lots of similar questions, but 1) none with a definitive, technical, thorough answer, and 2) none which relates specifically to EFB and AGM batteries.

The owner's manual doesn't say anything specific - it only says that I should bring the car to an authorised dealer to have the battery charged because they have the right equipment , which is ludicrous!

Any ideas? Thanks!

  • Perhaps the manufacturer specifies getting the dealer to do the charging as owners have made expensive mistakes, don't understand the technology or instructions etc...
    – Solar Mike
    Apr 10, 2020 at 10:13
  • Perhaps. Or perhaps because they want to overcharge for the most banal things. Just a thought :) Apr 10, 2020 at 10:29
  • Having had to replace alternators on customer's cars because they did not follow instructions I know which I consider the most valid...
    – Solar Mike
    Apr 10, 2020 at 10:32
  • Fine. However, with the world under lockdown and the dealers closed, going to a dealer is not an option. Can you please help with the original question? I charge the battery of my motorcycle at home and the thought the car manufacturer refuses to provide any detail is, honestly, shocking, but whatever. Having been ripped off by dealers I know which I consider most valid! Apr 10, 2020 at 10:45
  • My garage is not closed - drop the car off and they work on it... but you don’t get to get close to them or watch them.
    – Solar Mike
    Apr 10, 2020 at 10:51

4 Answers 4


As long as the charger is rated for EFB, you don't have to worry about overcharging while in the vehicle. It's the older chargers that could output up to 18 volts that can fry electronics. Those should be junked.

  • So what matters is ensuring that amperes and volts stay within a certain range, something which newer chargers do ensure but older ones don't? How about charger types? Some support AGM but make no mention of EFB, would they cause damage and how/why? Apr 11, 2020 at 7:57
  • 1
    Correct. Contact the charger manufacturer and ask them what settings to use for EFB.
    – user9181
    Apr 16, 2020 at 13:58

In addition to limiting max voltage, EFB and AGM batteries require different recharging protocols. It's not like the old days. Now you must match the charger to the battery or you can damage the battery.

  • But does EFB require a different protocol from AGM? Also, what does different protocol mean? How many ways are there to recharge a battery by supplying a certain quantity of amperes? Apr 11, 2020 at 18:50
  • Unfortunately this isn't easy to answer without getting technical but basically every different type of battery requires a different "protocol" to be charged. Protocol can encompass many things for example the number of amps delivered at a certain charged percentage, fully charged voltage, fully discharged voltage, etc.
    – Techlord
    Apr 15, 2020 at 20:12

A battery charging protocol defines a certain voltage and amps for a certain period of time, then raising voltage but lowering amps until finally tapering volts to prevent battery plate overheating. Older battery chargers just blasted in a set number of amps during the entire charging period. I don't think the protocol is the same for EFB and AGM. But you'll have to contact the charger manufacturer to check on yours.

I would like to understand if I need to disconnect the battery before charging it. If so, both cables or just the negative?

Electrically, it does not matter. IF you want to disconnect, both cables, or just the negative is fine, but please don't disconnect just the positive because if you accidentally misplace the wrench between positive and chassis (ground), you can hurt yourself and/or the car by creating a short circuit. So if disconnecting both cables, disconnect negative first. And connect negative last (reverse order) when putting back the cables.

But, I would not disconnect. If connecting the charger properly, there's no room for destroying anything expensive. In fact, with a modern charger, you'll find the charger has a microprocessor that prevents you from destroying anything expensive. So, wrong voltage, reversed cables don't today always mean damage.

If you disconnect, you'll lose the ECU memory (might have a rougher / less fuel efficient engine for a while, can't take the car to inspection as it'll have to relearn the emissions info). You will also lose the radio presets.

In fact, some chargers have a "battery change" mode that continues to supply current when the battery disappears from the circuit. This mode allows you to change the battery without losing radio presets. (Please don't just disconnect the battery if you decide to change it; look at the charger specs first to see if it supports this kind of mode and how to use it; the battery is the buffer for the car's electrical system and with a wrong type of charger, you could have expensive damage if there's no buffer.)

Nevertheless, even though disconnecting the battery is not recommended, there's absolutely nothing wrong in doing it. For example, if the car airbags are worked on, the garage WILL disconnect the battery. So, sometimes you will need to reset the radio presets and the car will need to relearn the ECU info.

  • Thank you, very detailed. I understand that some chargers might try to "revive" an almost dead battery increasing the voltage, but too high a voltage could damage the ECU. However, most modern chargers should recognise when the battery is plugged in and, if that's the case, not increase the voltage to dangerous levels. Apr 24, 2020 at 20:32
  • Alos, on top of losing the ECU memory, there's a chance the battery needs to be programmed / coded again if it gets disconnected, especially if it is for a start/stop car Apr 24, 2020 at 20:33
  • To how high voltages does the "revive" attempt go? If the charger has a "revive" functionality, you might want to avoid using that for cars that have a totally dead possibly broken battery, or then turn off the "revive" functionality. I wouldn't buy a charger that won't allow me to turn off the "revive" functionality.
    – juhist
    Apr 24, 2020 at 20:36
  • I remember a Ctek which revives up to 16ish V, and an Optimate which can reach up to 22V, but only if the battery is not connected to the vehicle (how they recognise whether it is I don't know). I have no idea what Noco chargers do. Apr 24, 2020 at 20:39
  • 16V won't destroy anything, 22V could be a bit too high if the battery is not fully healthy (which if when you'll use the revive functionality anyway). I would stick with a good brand. Ctek is a good brand, and you won't go wrong by choosing a Ctek.
    – juhist
    Apr 24, 2020 at 20:41

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