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Q & A

Battery Charging

Dear Sir/Madam,
I have brought a CTEK MXS 5 battery charger this last year. I liked the compact size and its 8 stage charging. I do not think it can charge a battery bank x3 85 Amps = 255 amps(12v), as its output is up to 110amps and because of its testing would not work for more than one. I could not see in the Ctek range if they do one for 3. I have no one to tern to for help. I have found it hard to find information that is linked to chargers to buy. I have a 28 foot plastic boat.

Asked by: Gary  | 1.53pm, Friday 26 June

WW says:

CTEK do make a range of chargers, which include the battery bank size that you are using... like the MXS25. If you look through the CTEK site http://www.ctek.com/gb/en/chargers/12v there are quite a few different units
However, there are quite a few good chargers with multistage regimes, as well as the CTEK range.
Be aware that if you are making the installation permanent or even semi permanent, try and avoid using crocodile clip type connections and use the proper attachments to battery terminals (and positive and negative at opposite ends of the batter bank) to minimise both sparking and ensure good electrical connection.
Bear in mind that some of the "high voltage" battery regeneration settings on some battery chargers (including some CTEK models) produce a very high voltage, even if for a short period of time, and if equipment aboard is connected while this setting is chosen, damage might happen to some sensitive equipment.

Mark Langley  | 7.13PM, Friday 26 June

The 5A charger you have will not charge your batteries properly. It will not be able to advance beyond the charging stage into the conditioning stages.
It would be better to fit a bigger permanent battery charger which are available with seperate outputs to keep engine start and cabin batteries charged.
As Mark says; any battery connection must be permanant and croc clips are not permitted by the Boat Safety Scheme.

Rupert Smedley  | 11.25AM, Monday 29 June

It is not normally a good idea to join together batteries of different capacities together in one circuit- as the batteries have different internal resistances. In theory, they could accept the same rate of charge and stop when "full", but experience shows that this does not happen in practice, with the resistance of the smaller battery as it becomes charged preventing the larger cells from reaching their capacity. Also, the combined action as the cells decay with age becomes greater.
An alternative would be to switch the batteries so that you have two banks- the primary one of say two 110Ah and a reserve bank of 85Ah. You could connect the battery charger through a two-pole switch to select which bank to charge- which would then give you options to use one bank and charge another.
On cruisers with outboard motors, this can be advantagous (and those with small inboard engines as well) as, if you install two battery selector switches, one on the output of the engine charging coil, so you can direct the power to either (but not both) battery banks, you can then draw from the other bank with a selector switch in the domestic draw off. This works very effectively and I have done several installations like this.

Mark Langley  | 10.36AM, Tuesday 30 June

Readers say:

Re: My CTEK MXS5.0, Yes I understand as said in my Q. (may be not so clearly), I would have to charge each battery independently and not connected to remainder.
Thank you for your answers.
Always Open for charger recommendations for 3x 85amps or 110amps battery banks if I upgrade in the future. May buy Ctek MXS 25. Rather expensive but maybe all makes are.
I was told from a battery supplier at Crick boat show this year, that each battery need not be the same amps (stored power) example 1 @ 85 amps and two @ 110amp all 3 being 12 volt in one bank. Would each battery just charge to its own capacity ?

Gary  | 4.26PM, Monday 29 June

Thank you for your answers Mr Langley & Mr Smedley. Thank you for the very good advice (11.36am 30th June)I will do just that and have 2 banks one in one and 2 in another for the domestic (for now all 85amp/hours) I now have 2 85 amp hours (12v) = both or one or other. It would be of great help for a wire diagrams in WW for what you describe (Mark Langley | 10.36AM, Tuesday 30 June)
Thank you one again well worth being a WW subscriber.

Gary  | 12.09PM, Wednesday 1 July

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