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Question about amps and mppt magic.


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My brain gets all fuzzy with numbers.

I currently have a 12v lifepo4 100ah battery.

it’s connects to a 100 watt panel that produces 18 v dc at roughly  5 amps.

it seems to take forever to charge my battery. So..

 

If I connect say a 400w panel that gives 45v dc with 10 amps make a big difference?

Is it amps that speed up the charging? I mean will my mppt step down the 45 v from the panel to say 14 for charging the battery then convert the leftover volts into amps? Thus increasing the amps. 

trying to figure how to best charge the battery quick from solar.

The mppt handles 55 amps

 

 

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Expanding on the answer above. With PWM the larger the mismatch in voltage between the panel & battery the less power the panel can deliver. So when the battery is most drained is when the charge rate is at it’s lowest (the opposite of what you want). In this situation of a low state of charge it’s like the PWM charger is trying to pull off in 3rd gear whereas the MPPT is an 8 speed automatic gearbox. Speed & Power ;)

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1 hour ago, sunset1 said:

t’s connects to a 100 watt panel that produces 18 v dc at roughly  5 amps.

it seems to take forever to charge my battery.

Ok, the charge controller should take the voltage and current, in other words, the power, produced by your solar panel and transform/convert it to the right voltage and with low loss, to the maximum amperes it can still extract... you say 18V X 5A that would be 90W produced, but you should not charge your 12V LPO battery with 18V, probably more like 14V would be ok (possibly 14.4V), either way, the 90W/14V would then be nearest to 6.4A into the battery, which if the battery is sitting at nearly 0% capacity remaining (flat battery), then you'd would require 100(Ah)/6.4A=around 16 hours to recharge the battery to full...

The 100Ah 12V battery could possibly be recharged from empty to full in 1 hour, by providing it 100A for an hour at 14V or so, but check the specs of the battery, either way, I would not suggest this high a charge/discharge rate for such a long period of time, better for the battery's life would be to say, charge it back to full over 2 or 3 hours by letting the charge current sit at 50A or 34A instead, also realise the cable diameters you'd need to run at the higher current.

Either way, I hope you are not connecting the battery straight to the solar panel(s) but are using a suitably specced charge controller between the panel(s) and battery.

As for the 400W panels with 45V and 10A, that would be a 460W or larger panel, since 45V X 10A = 450W and you'd be unlikely to get the panel to produce more than its rated for or even close to what its rated for... but if your panels produced 45V and 10A and your charge controller could manage this, then it should provide 32A or so at 14V to charge the battery and this would put you in the 3 to 4 hour range, assuming the battery is totally discharged, which again, you would be wise to avoid, down to 20% of charge remaining is probably ok, but totally flat is going to impact the battery's life negatively.

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2 hours ago, Vassen said:

Hi

So the difference comes in with the mppt. There are 2 types of charge controller, pwm  and mppt. 
 

PWM will basically drop the panel voltage to match the battery voltage. This means that the panel is no longer operating at its Vmp, which also means that the current of the panel will drop. It becomes more apparent if there’s a large mismatch between the battery voltage the panel voltage. 

A mppt on the other hand will basically keep the panel operating at its Vmp, which means that the panel produces the maximum power. This Power is then converted to the voltage of the battery and it’s applicable current based of the formula I = P/V minus some losses. 
 

The higher current will definitely help to charge the battery faster but it also depends on the battery specifications as to what the max charge rate is  

 

Hi thanks the max charge is 0.5C so guessing 50 amps.

For instance would the battery charge faster at 15v and 50amps or 80v at 6 amps?

 

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5 minutes ago, sunset1 said:

For instance would the battery charge faster at 15v and 50amps or 80v at 6 amps?

You should not and can not charge the battery at a higher Voltage than specified by the manufacturer, for a 12V replacement, which is what you battery is, most likely, 14.4V should be the maximum Voltage you should ever try to charge at.

The time it takes to charge would then be determined by the current (Amperes) that is provided to the battery. If the max charge rate is 0.5C then 50A is it and for longer life expectanct try 0.3C or 30A, which would then be under 4 hours still to recharge from totally flat, which you hopefully won't be doing... how fast do you need to recharge and how deep do you discharge the battery?

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2 hours ago, Kalahari Meerkat said:

Ok, the charge controller should take the voltage and current, in other words, the power, produced by your solar panel and transform/convert it to the right voltage and with low loss, to the maximum amperes it can still extract... you say 18V X 5A that would be 90W produced, but you should not charge your 12V LPO battery with 18V, probably more like 14V would be ok (possibly 14.4V), either way, the 90W/14V would then be nearest to 6.4A into the battery, which if the battery is sitting at nearly 0% capacity remaining (flat battery), then you'd would require 100(Ah)/6.4A=around 16 hours to recharge the battery to full...

The 100Ah 12V battery could possibly be recharged from empty to full in 1 hour, by providing it 100A for an hour at 14V or so, but check the specs of the battery, either way, I would not suggest this high a charge/discharge rate for such a long period of time, better for the battery's life would be to say, charge it back to full over 2 or 3 hours by letting the charge current sit at 50A or 34A instead, also realise the cable diameters you'd need to run at the higher current.

Either way, I hope you are not connecting the battery straight to the solar panel(s) but are using a suitably specced charge controller between the panel(s) and battery.

As for the 400W panels with 45V and 10A, that would be a 460W or larger panel, since 45V X 10A = 450W and you'd be unlikely to get the panel to produce more than its rated for or even close to what its rated for... but if your panels produced 45V and 10A and your charge controller could manage this, then it should provide 32A or so at 14V to charge the battery and this would put you in the 3 to 4 hour range, assuming the battery is totally discharged, which again, you would be wise to avoid, down to 20% of charge remaining is probably ok, but totally flat is going to impact the battery's life negatively.

Hi thanks that explains a lot.

We have the mecer ivr1200mppt. Has a 55 amp limit on the mppt and 100 max v for dc.

 

With regards to the 400w etc panel and the conversion to 32A at 14V roughly, would the mppt be responsible for the conversion? 
we have 2.5 mm2 wire apparently goods for 20amps from panel to inverter/ mppt and don’t want to burn our house down.

hope this makes sense.

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3 minutes ago, Kalahari Meerkat said:

You should not and can not charge the battery at a higher Voltage than specified by the manufacturer, for a 12V replacement, which is what you battery is, most likely, 14.4V should be the maximum Voltage you should ever try to charge at.

The time it takes to charge would then be determined by the current (Amperes) that is provided to the battery. If the max charge rate is 0.5C then 50A is it and for longer life expectanct try 0.3C or 30A, which would then be under 4 hours still to recharge from totally flat, which you hopefully won't be doing... how fast do you need to recharge and how deep do you discharge the battery?

TX, sorry i miswrote earlier. The 17v was the reading from the solar panel. The voltage going to the battery however never really goes over 13.9 as of yet. I’ve only had this setup for a few days.

we would like to recharge the battery in 3 to 4 hours. I’ve set the discharge levels to 20%, and full charge to 90%

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17 minutes ago, sunset1 said:

With regards to the 400w etc panel and the conversion to 32A at 14V roughly, would the mppt be responsible for the conversion? 

Yes, the panel will feed whatever its Voltage is to the charge controller/inverter/whatever you are using to charge your batteries with and this unit would then, if its an MPPT do a DC to DC conversion from the voltage supplied by the panel to the correct voltage to charge the battery, but this conversion should occur with fairly low losses, give you more current (Amps) on the battery side, than what the solar side is presenting.

Having written this, please ensure that the panels are not generating a higher voltage than what the charge controller can manage, basically a lot of folks around here are running their houses off 5kW and larger inverters, which can happily use 200 even 300V or higher at relatively low current, 8 or 9Amperes to charge their 48V battery banks and generate 220V otherwise form the excess power produced by the solar panels, basically you need to know how many Volt the charge controller can utilise on the input side and ensure that you do not exceed this, else the magic smoke may come out 🙂

Edited by Kalahari Meerkat
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8 minutes ago, sunset1 said:

recharge the battery in 3 to 4 hours. I’ve set the discharge levels to 20%, and full charge to 90%

Those are reasonable numbers, I reckon and 70% or around 70Amperes in 3 or so hours, would require less than 30Amperes from the charge end, can your charge controller provide, let's say 30A to charge the battery with? You'd need around 400-odd W on the solar panel end to make this work.

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18 minutes ago, sunset1 said:

Hi thanks that explains a lot.

We have the mecer ivr1200mppt. Has a 55 amp limit on the mppt and 100 max v for dc.

 

With regards to the 400w etc panel and the conversion to 32A at 14V roughly, would the mppt be responsible for the conversion? 
we have 2.5 mm2 wire apparently goods for 20amps from panel to inverter/ mppt and don’t want to burn our house down.

hope this makes sense.

Watts = Volts x Amps

If your battery charge limit is 50A at 12V that is 600W max solar panel you should use.

In the case of your 450w panel, the mppt will convert your 45V x 10A into 13.6V x 33A.

So between your panel and your inverter you would need 10A cable. From your inverter to your battery you would need to take your inverter power (I'm assuming 1200W) and divide that by your battery voltage (which is 12V) to get 100A cable.

Edited by tetrasection
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