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Is this Gennysize/batterybank/charger correct?


Tara1234

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Hey so I had a look at my genny yesterday and found that it is a 850W petrol frame type. 

My battery bank is 450AH and im thinking of getting a 50amp charger. Have I done my maths correct?

50 amp charger X 12v = 600W

850W *0.75 (3/4 load) = 637.5W

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No - batteries charge at 13.5 to 14.5 volts so your watts are a bit low. Well done for doing the power factor calculation though.

50 X 13.5 = 675

I suspect it will be a bit marginal. I also doubt that you would notice a significant difference in overall charge to fully charged time if you bought a30 to 40 amp charger. Probably half an hour or so.

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I think you might struggle to run a 50amp charger too. 

50 x 14.8v (wet lead acid batteries?) x 1.15 (inefficiences in the system) = 851w. 

If the stated 850w is the peak generator power rather than the continuous rated load then it won't work. You either need a smaller charger or a bigger generator.

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30 minutes ago, Tony Brooks said:

Well done for doing the power factor calculation though

I'm not sure the OP has taken the Pf into account.

I think she has 'just' taken the generator output and decided that its better to run it at 3/4 load rather than 'full load'

 

But - I may be doing her an injustice.

 

16 minutes ago, blackrose said:

50 x 14.8v (wet lead acid batteries?) x 1.15 (inefficiences in the system) = 851w. 

Surely by the time it 'gets to' 14.8v it will not be charging at 50 amps (maybe 8-10 amps max)

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3 minutes ago, Alan de Enfield said:

 

Surely by the time it 'gets to' 14.8v it will not be charging at 50 amps (maybe 8-10 amps max)

Dont chargers charge at their max Amps for the whole of the bulk charging phase?

i.e. In the OP case, the voltage might start at say, 13V, and rise to 14.8V, all the time charging at 50A. Thus the power before inefficiency would be 740W which may be a bit too close for comfort.

I have an 80A charger and, at start up, it always rises to 80A in a second or so.

 

In the OP case I'd probably play it safe and get a 40A charger, or buy a more powerful genny - perhaps with a continuous rating of 1Kw or more.

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10 minutes ago, Richard10002 said:

Dont chargers charge at their max Amps for the whole of the bulk charging phase?

You cannot 'have' high amps AND high voltage.

The charger will start at (say) 13 volts (it could even be low 12's) with 50 amps, but as the SoC of the battery increases it will 'control' the charger.

As the voltage increase to towards the 'high' 14s then the current, by default* must reduce, which is how you can tell when a battery is fully (whatever that means) charged - the voltage will be 'high' and the current around 1%-2% o the battery bank capacity.

 

* as 'watts' (w) are fixed then the variables are current (a) and voltage (v)

W=A x V

So if V increases then A must reduce.

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I had a 650w petrol genny to power a 24 amp (at 24V) charger. It coughed and spluttered and charged exceedingly slowly (slowly as in paint drying).

I suspected that the genny's voltage dropped  considerably. I gave up and bought a girt big 2600W genny which was slightly more successful.

Conclusion: It's not the best solution........

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5 hours ago, Tara1234 said:

Hey so I had a look at my genny yesterday and found that it is a 850W petrol frame type. 

My battery bank is 450AH and im thinking of getting a 50amp charger. Have I done my maths correct?

50 amp charger X 12v = 600W

850W *0.75 (3/4 load) = 637.5W

Solar panels are quiet and maintenance free.  A portable petrol generator and fuel have to be stored in a ventilated metal container for safety and to comply with Boat Safety.

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1 minute ago, mango said:

Solar panels are quiet and maintenance free.  A portable petrol generator and fuel have to be stored in a ventilated metal container for safety and to comply with Boat Safety.

See Taras other thread. 

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2 minutes ago, mango said:

I see – not enough space.  But there is still the problem of safe storage of the generator.

I think that is one of the points chubby made in the other post, not sure what the reply was 

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14 minutes ago, rusty69 said:

I think that is one of the points chubby made in the other post, not sure what the reply was 

I dare say I should read all the posts before commenting but would not want to start a trend.  :-)

  • Greenie 1
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47 minutes ago, Neil Smith said:

If the fuel and genny are not on the boat when the bss man comes it will pass.

Neil

And if they are found by the insurance bod after your boat is gutted by a fire thats not even got anything to do with the petrol gennie do you think they will happily pay up?

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Frame genny will likely have very poor regulation and may blow up an electronic multistage charger.

ISTR Screwfix do a little 700W Impax inverter genny for £200ish, should run a small cheap 20A charger OK:

http://www.screwfix.com/p/impax-im800i-700w-inverter-generator-230v/15760

Here's a typical inexpensive 20A charger:

https://electroquestuk.com/chargers/leisure-battery-chargers-marine-battery-chargers/fully-automatic-marine-leisure-battery-charger-12v-20a

With solar it won't be needed til late autumn anyway. That genny/charger combo should cover power use from some LED lights, water pump, phone charger and modest lappie use.

If you're London based it'd be well worth checking out the London Boaters facebook page, see what other people run for a similar sized boat.

Edited by smileypete
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  • 2 weeks later...

Thanks for your help people. Also in regards to charging my leisure batteries. I have heard that they charge slowly after a certain percentage (think it starts at 50% and gets mega slow after 80%).

So should I charge them all the way to 100% or could I get away with charging them up to 80% and discharging them to say 30% rinse and repeat or do I have to keep them at 100% to avoid damaging them. I imagine its a lot cheaper to keep them between 30% and 80% as they will charge at the rated ampage of the charger.

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5 hours ago, Tara1234 said:

Thanks for your help people. Also in regards to charging my leisure batteries. I have heard that they charge slowly after a certain percentage (think it starts at 50% and gets mega slow after 80%).

So should I charge them all the way to 100% or could I get away with charging them up to 80% and discharging them to say 30% rinse and repeat or do I have to keep them at 100% to avoid damaging them. I imagine its a lot cheaper to keep them between 30% and 80% as they will charge at the rated ampage of the charger.

You should charge them to 100% to prolong their life, and avoid sulphation. 

 

If you discharge to 30% and then charge to 80% you will seriously reduce their lifespan,assuming you have lead acid batteries. 

 

 

 

Edited by rusty69
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To explain a little.

If a lead acid battery is left with any degree of discharge it will start to sulphate and the longer it is left before recharging the harder the sulphate on the plates will become until eventually you can not convert it back to a lead oxide by charging. This gradually reduces the battery capacity. So ideally you should recharge as close to 100% as practical the moment you discharge the battery. In practice once a day is probably OK. Charging to 100% with anything but wind or solar is probably not cost effective so many people go to about 80% for 6 days and the charge to 100% once a week and seem to get good battery life.

Lead acid batteries have a certain design life in discharge-recharge cycles known as the cyclic life. To get the most out of whatever cyclic life the batteries have most people try not to discharge below about 50% on a regular basis. Taking a very rough and probably slightly inaccurate view of the situation if a battery has a 150 cycle life if you only discharge it to 50% you actually get 300 cycles to 50% discharge so giving a longer life.  This is only an illustration to give you an idea about all this. Nothing is as black and white. very occasionally discharging to below 50% is unlikely to have a measurable effect on battery life but doing it regularly will.

I would point out that my Exide wet open cell leisure batteries are quoted as having a cyclic life of 300 cycles to 40% discharged (that is 40% of fully charged). You are proposing going to 30% which is below Exide's recommendation so you will experience a shortened battery life.

Stick with charge to 80% each day, as close as possible to 100% once a week and do not discharge below 50% whenever possible.

 

PS how are you monitoring the state of charge?

Edited by Tony Brooks
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