WattFlyer RC Electric Flight Forums - Discuss radio control eflight - View Single Post - Operating my LiPo Charger from batteries
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Old 05-31-2019, 11:21 AM
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Join Date: Oct 2010
Location: Ex UK Brit now in Latvia west coast - Ventspils
Posts: 12,370

OK ... yes you can charge using a battery as power source. But your assumption on ampage is not enough.

Remember that as the battery powers the charger - it is also reducing its own level of charge. Any Lead Acid battery - even a Deep Cycle does not appreciate being driven lower than 50% charged level.

Given that a 3S LiPo is similar voltage level to a reasonably charged Lead Acid battery - we can then base on the ampage needed ... but taking into regard the 50% mark.

You wish to charge up 9x 3S 2200 packs. .... that's 9 x 2.2 as you rightly say. ~20A/hr.

Allowing a 50% mark ... now the power supply battery needs be 2 x 20A/hr = 40A/hr

Now we have to adjust because at 50% charge level - the Lead Acid battery will be lower voltage and struggling to provide the WATTS needed for the charger. I suggest add another 25 - 30% capacity ... now we are looking at a Lead Acid battery of about 50 - 60A/hr.

Lets move to practical and leave the techno aside.... I have always maintained that if you want to use Lead Acid batterys - then get to a Vehicle Scrap Yard where they sell of 2nd hand car / truck batterys. They will usually load test the battery in front of you ... cost is far less to pay .... and if it doesn't do the job - you haven't lost much.

My preference is for batterys supplied to power vans / trucks with hydraulic tail-lifts. They are a compromise battery that can suffer 50% discharge better than usual car battery.

Beware of usual so-called Deep Cycle batterys - because most are actually compromise format such as needed in RV's, Boats to power domestic as well as start engines. True Deep Cycle batterys should not be used for engine starting.

I have boats and I stopped buying Leisure batterys many years ago and now buy Scrap Yard batterys ... present batterys are 2 years old in my yacht ..

Final comment : You can while driving to flight location - charge your Lead Acid battery using your B6 ... so you know its topped up before swapping it round to power the charger !
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