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hi everyone jeff cote here with

boatingtechtalk.com we've got a question

from a fellow boater alex asks

jeff connected to shore with full

12-volt lighting and electronics running

okay you're right so a lot of us do that

right we're we connect to shore power

and we have a lot of things that are

turned on right the lights might be on

the fridge might be on maybe the nav

right underwater lights whatever by

stereo you've got most of us when on our

boats connected to shore power are

actually using 12 volts

now here's the question is the battery

charger that's connected to battery bank

supposed to sufficiently keep up with

the batteries when under load

and not let the batteries drain

question mark

follow-up question any workaround to

using 12-volt systems that don't affect

your banks by drawing too much

it's a great question from alex the

reality is that that is often overlooked

on too many boats

think about it

right so let's say for example you've

got a 40 amp battery charger right or

even a 20 amp which is actually common

so let's actually use a 20 amp battery

charger because it actually applies to a

lot of boats you've got a 20 amp or a 30

amp but 20 amp is a good example you've

got a 20 amp charger

and now you connect to shore power and

that 20 amp charge is going to charge

just two batteries

but the issue is when you're connected

short power you're also deciding to run

some loads on your boat your

refrigerator on the lights are on chart

plotter's on so you could easily find

yourself easily drawing 15 amps and

that's not without a lot of effort 15

amp draw is not a lot

right especially if you're awake and

it's maybe dark outside and you decide

especially with illumination

so now what's the net going in your

batteries you have a 20 amp charger but

you're using 15 amps so your battery

charger is outputting 20 you're using 15

and you're only netting five

so now five amps going in a battery bank

and let's assume that battery bank is

200 amp hours because generally

bulb builders always size a battery

charger to be 10 percent

of battery capacity

so if they put a 20 amp battery charger

there's probably a 200 amp hour battery

bank now if you've got a 200 amp hour

battery bank and it's somewhat depleted

half of 200 is 100 now now here's the

sensor right what happens now you've got

you're only charging at five amps

because you're using 15. so it's gonna

take you five into a hundred it's going

to take you 20 hours to recharge that

battery bank because when you're on the

boat and using the boat you're taking

away the charging amps

for your own benefit

right

so that's why some boaters literally and

i've seen it they had to literally get

off the boat they couldn't be on the

boat to get a charging

worst example i've ever seen or best

example 800 amp hour battery bank with a

20 amp battery charger on a grand bank's

42. when they would actually connect to

shore power at a marina they would

actually deplete their batteries their

only way for them to recharge that boat

was to leave the boat and turn

everything off

because by the time the refrigeration

was running and everything else

they were barely keeping afloat right

and so that's the reason why it's really

good idea to always have a little bit

bigger charger you know the minimum

is not

most often the desired state it's the

bare minimum so when you're sizing a

charger for your boat especially when

you're in the lower amperage and you're

doing minimum and you might have only a

20 or 30 amp charger

think about what is going to be a

typical current draw that you're going

to do when you're on the boat and you

want to recharge the batteries at the

same time

and generally that means sizing a

charger especially on the lower lens not

going with the minimum if you need a 20

get a 30 if you need a 30 get a 40 right

now i'm not saying if you need 100 get a

150 because at that point you're fine

you're not going to be drawing 100 amp

dc loads for most of us right but you

know

tens 20s 30s you know could easily be

40s right not always but if you need a

10 maybe get a 20. go a little bit more

up so that you can offset your use of

the dc power while you're on board

whilst at the same time recharging your

batteries

and so alex actually entered that and he

said perhaps a higher output charger

maybe and he's bang on that's exactly

right go with a bigger charger so that

you can both charge and run your loads

at the same time

great question alex thanks for asking

if you're curious we've written whole

articles about this go on our website

search it out

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