What size battery charger

[OutOfTheCloset]

Having just got my boat it came with a dead 70amp battery charger. As I only have a few lights/fridge and a water pump I think that 70amps is a little over the top.

Looking into what size to get I believe that most shore supplies are rated at 16amps and wondered what's the benefit of having a charger rated higher than that.

Thanks in advance

 

[TargaLout]

16A Shore power is the 230v AC current available to your boat so you can run things like chargers etc. A 70 Amp battery charger is the maximum current it can supply to your flat battery. So if you had a flat 220A Hour battery it should take 3 1/4 hours to charge it up from a 70Apm rated charger. (220/70=3.14 approx).

 

[temptress]

don't get confused over AMPS, watts and wolts

Put simply ( I know thereis more to it but .....)

Watts = Volts X Amps. Your electrical equipment is usually rated as WATTS. Although some marine equipment also gives Ah (Amps user per hour) as a rating.

So at on shore power you can use up to (in theory): 3840 (watts) at 240v and 16a

on batteries at 12V you get 840 watts (12 x 70) or on 24v it's 1680watts (24x70). There are other factors but you generally need a big a battery charger as you can run to chage the batteries and run whatever equipment you are using at the same time.

 

[Talbot]

There is a recommended relationship between charger size and battery capacity. - hopefully somebody will post the details cause I am having a senior moment.

 

[TrueBlue]

There's an awful lot of muddled thinking and practical inaccuracies in this post, but then electrics always cause confusion. Dunno why. Here's some approximate thoughts.

It's considered best practice NOT to charge a lead acid battery at more than the "five hour rate". That would give you 220/5 = 44 amps. So a 70 amp charger would not do your batteries a lot of good IF it ran at full chat regularly. A modern multi step charger wouldn't charge for any length of time at its full rate anyway.

A flat battery is not actually at zero volts (at least I hope - 'cos it would be irretrievably dead) so 220/70 = 3.14 hrs is misleading; more likely to be a couple of hours due to the stepping process.

What really matters is what do you want the charger to do. If you are using the boat at weekends, then I suggest a small simple 20A charger from Sterling ('cos they work and are inexpensive) will do the job. Your evening use of the facilities would be recovered overnight, and if your use is a bit in excess the charger has all week to top the system up.

Think on this if you are actually using 20 amps when you're onboard (and you're probably not) then methinks 220 amp hours is way too small a capacity anyway.

Hopefully, proper experts (I'm just a practical guy) may correct and / or amplify what I've said.