Maintenance charging battery

Faults and Technical chat for the Discovery Sport
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Re: Maintenance charging battery

Post by Canuk » Wed Sep 18, 2019 4:54 pm

There is lots of foam around the exit in my battery box so just ran the wires using the foam as protection. The lead has a fuse so even if it shorts out because of poor installations it will not harm the car.

Keep the cover on ,,keeps debris off the battery. But then for years batteries had no cover.
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Re: Maintenance charging battery

Post by Mamil » Thu Sep 19, 2019 4:05 am

Just wanted to add a few clarifications to the information already on this thread...

1. There is no problem charging the DS battery in-situ with the small charging current used by a CTEK 5 charger. LR are just playing safe suggesting you remove the battery, and it's not an easy process, and requires some things re-setting on replacement so is a PITA.

2. No I wouldn't drive around with the battery cover off. AGM batteries are more sensitive to heat than wet cells, and can be cooked much more easily by the heat in the engine bay. That's why in the DS they are effectively in a separate compartment that has the air going to the internal air vents flowing over them. If you leave the cover off you are exposing them directly to the heat from the engine bay, and incidentally sucking in hot engine bay air to your ventilation system.

3. There is a sensitive bit of electronics called the BMS, attached directly to the negative terminal of the battery. This measures the flow of current in and out of the battery and provides this information to the car's ECU so it can calculate what state of charge the battery is at. If you connect the charger, or anything else for that matter like an accessory, direct to the battery negative post you are bypassing the BMS so it's calculation of battery state of charge will be inaccurate. That's one reason for connecting to the chassis ground point instead, or at very least to the non-battery side of the BMS. Second reason is that any voltage surges (sparks!) can easily damage the sensitive BMS. Not such a problem with the CTEK chargers because they are well protected, but other chargers and especially jump cables for jump starting can cause voltage spikes as you attach them which can damage the BMS. That's why it's safest as a general rule to only connect things to the chassis ground point on modern cars, not the negative battery terminal.

4. While it's OK to attach a charger at other points in the system, rather than direct to the battery, there can be issues. The accessory sockets/cigarette lighters on the DS are disconnected by a relay after a certain time the car is inactive, so are no use for charging. You need an always live direct connection. The trailer socket has such a wire, but it's a long way away from the battery and connected by a small gauge wire (2.5mm2), and there will be voltage drop as a result (the voltage at the end of the wire is not the same as what is put into it, the difference being lost as heat along the length of the wire). Why is this an issue - well the CTEK is quite a sophisticated charger and it relies on knowing the exact voltage of the battery at any point in time to decide on it's charging regime. If it's not sensing voltage accurately due to voltage drop it will not charge optimally. Also, it uses temperature compensated charging, ie. reduces the charging voltage if the battery is getting hot to avoid damaging it - especially important with AGM, as already mentioned above. If the charger is at the back of the car it won't be measuring accurately what the temperature of the battery is.

5. So, bringing all this together, the optimum solution is to connect charger by shortest and most direct route possible to the battery positive, and the chassis ground point (just behind the battery). Which is how I've wired mine and brought the socket out right next to the battery box so I don't have to take the lid off....

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Re: Maintenance charging battery

Post by Lofty1 » Thu Sep 19, 2019 10:28 pm

Thanks Mamil your advice is much appreciated.👍
MY2017 Discovery Sport 180 HSE auto in corris grey with black roof, privacy glass, tow bar kit.
Also 2012 BMW 330i Touring
Previous 2008 Freelander 2

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