Charger question - Max current

[Jussi]

So I'm thinking about getting a Revolectrix Powerlab 8 and went to their site to figure out what power supply I needed. They have a calculator that's supposed to help with this but I'm a little confused on some of the parameters. Can someone clue me in.

http://www.revolectrix.com/pl8_calculators/PL8_Calc_2.htm

1. I'm unclear on how I determine the value for the questions.
"What is the minimum power supply I can use to achieve this output current? "

"I want to charge at the following maximum current: ?"

2. I don't know understand how to read the answers. I thought it would give me a single Amp and Voltage value but it gives me a string of them.


http://www.revolectrix.com/pl8_calculators/PL8_Calc_2.htm



And if you guys use a Powerlab 8, what power supply do you use?

 

[crayfellow]

The opportune question for you is, what cell count and what capacity batteries do you intend to charge now and in the foreseeable future?

If you can answer that question it will be very easy for me to help you pick a power supply.

They are giving several voltage/current values since a given power supply will be able to provide this much current:
<stated supply power in Watts>/<house AC voltage>

...but then in order to figure out how much current this will equate to on the charger, you need to factor in the cell count of the battery you intend to charge since <voltage> * <current (amps)> = <power (watts)>, along with the capacity of the battery and the rate you want to charge it at. Just as an example a 6S battery is 25.2V. Let's say you have a 10Ah 6S battery you want to charge at 1C. This will require 25.2V * 10A or 252W. Then you need to take the output voltage the supply will provide in order to figure out how much current it will need to be able to allow for. Let's assume it is supplying the charger at 24V. 252W / 24V = 10.5A. We haven't even accounted for the efficiency of the supply, since some of the energy is lost in the form of heat as the AC house power is converted to DC to supply the charger, and you want to include that in the calculation as well.

Long story short, if you're getting into big, powerful chargers like that you either want to order a big, powerful supply (such as a 1200W unit from progressiverc) or if you have the time and like to tinker, I can suggest some HP computer power supplies you can modify into an equally powerful supply for a fraction of the cost (assuming your time is free, which obviously is not the case, but you need to factor time into the cost....most people don't bother to do that).

Cheers,
Patrick

 

[Jussi]

Thanks for the detailed explanation Patrick. I just ordered my first lipo which is a 4500mah 6s. I will prob experiment with higher capacity packs in the future but doubt I'll go with more cells.

I've noticed some people selling modded server power supplies on eBay. This one caught my attention but it only supplies 900w at 120v and 1200w 240v. Will I be missing out since the powerlab is capable out 1344w.

http://www.ebay.com/itm/12v-power-s...aultDomain_0&hash=item27ff571532&_sid=2048038


Also is there a way to make the charger portable? Will it work on a car cigarette lighter?

 

[crayfellow]

Let's say you're charging 2 of those 4500mah 6S batteries in parallel (using MPA or a ParaBoard, be sure to get the connectors you need, and if you get a ParaBoard since it has more connector flexibility, make sure to get a balance adapter to go between the FMA balance plug on the charger, and the ParaBoard... and only ever charge batteries of the same cell count and similar level of discharge). If you want to charge them at 1C, you need 4.5A x 2 or 9A at 25.2V. This only requires 226.8W from the power supply, a small fraction of the 1344W the PL8 is capable of. at 2C for those two batteries in parallel you'd need 9A x 2 or 18A at 25.2V, requiring 453.6W.

I would add that if you're only ever going to charge 6S or smaller, get the cheaper PowerLab 6. It's essentially the same charger, nearly as powerful, and probably plenty for what you need. This is what I own since I have no intention of charging anything beyond 6S. Check the comparison page.

For that charger you will want to power it with something that can give you 24V. So when you're using 12V computer power supplies, use two and wire them in series. You need to take care to remove the AC/DC ground in one of them so it will "float" and give you 24V altogether. Two of these in series, for example, will give you all the power you need.

If you'd rather not bother with all of that, I know of two sellers who have these more or less ready to go: http://feathermerchantrc.com and http://www.2zo-rc.com .

The cigarette lighter will not provide enough current; in the field folks use a car battery. Be sure to check the manual to see how to set up the charger to be powered from a battery, set up a current limit, etc.

 

[Jussi]

What is involved in adapting the power supplies to work with rc chargers? I saw one of the vids on the site you linked saying you need a grounding strap. Is that necessary? Also I actually have 250V in my garage since some of my woodworking tools require it. Can I just one since will output 1200w at 240v?

Any experience using car batteries? How do you recharge them?

 

[crayfellow]

What is involved in adapting the power supplies to work with rc chargers? I saw one of the vids on the site you linked saying you need a grounding strap. Is that necessary? Also I actually have 250V in my garage since some of my woodworking tools require it. Can I just one since will output 1200w at 240v?

Here is a sample setup process for the HP supply, you will find bunches of these on eBay for around $25/ea, maybe even local to you:
http://www.rchelination.com/setting-hp-dps-1200fb-power-supply/

The key is "floating" the ground on one of the supplies, but keeping the AC/DC ground on the other, then they will give you 24V in series. The other thing with these HP ones is to use a resistor to fake the connector into thinking it's plugged into a machine, since there is a load there. Otherwise it won't know to start up.

Proviso here is: be careful, be mindful of using sufficient wire gauge and quality connectors (I like Anderson Powerpole with the Tricrimp crimper; the 45A version in an application like this will be in order), and check your connections with a multimeter to ensure no shorts and expected voltage before connecting your fancy charger.

240/250V - Sure, but you might not want to depend on that if you would ever use the supplies outside your shop. Ideally you'd build something that would be happy on 120V too.

Any experience using car batteries? How do you recharge them?

oddly enough you can use a Lead Acid program on a PowerLab 6/8 (but not while it's your power source ).

 

[crayfellow]

another thing on the 240V - they certainly will be happy powered by 240V and there's no reason you couldn't start with just one since it will give you all the power you need right now. You can always grab another in the future.

 

[Jussi]

Hmm with the cost of the crimper, it makes the cost close to the modified PS. Between the 2 which one would you recommend and why? Also what type of connectors do you use to connect to the powerlab. I believe the powerlab uses and EC5, so do you just stick with that? I assume these can run on 240V as well?

 

[crayfellow]

use whatever connector you like. PowerLab 6/8 come with EC5 on the supply end, so just use that, then on the battery charging end, just use whatever happens to be on your batteries if you don't have a preference. Batteries these days tend to have XT60 (up to 4S) or XT90 (6+S). I put PowerPoles on everything just because I prefer them personally.

If you don't think you will need to charge more than 6S then you really don't need to spend the extra money on a PowerLab 8. PL6's 1000W could parallel charge 4 6S 10000mAh batteries at 1C. Put the cash toward the power supply or some extra batteries.

Where you're concerned with the 240V is the power supply. The charger only runs on DC, so think 12 or 24V.

 

[crayfellow]

@Jussi what did you end up deciding on?

 

[Jussi]

While not absolutely set in stone I'm pretty sure I'm going with the PL8 and PS from 2ZO. The PL8 because while I doubt I'll be using anymore than 6S I'm very new to the hobby and think the extra $70 is worth a little future proofing. I was highly debating on the diy router for the PS vs the pre-modded kit but eventually went with the latter because after the tools and parts needed to do the mod I think the savings will only be about $30-$40 and in the end it wasn't worth it for me. Plus based on my emails with Mike from 2ZO he will do a a far better job than me with all the connections. Not to mention the testing involved. I had some unexpected car problems so I'll have to take car of that first . But hopefully by next month I'll be placing the order.

I will probably post follow up questions once I get the equipment. Btw, what type of charger are you using?

Thanks for all the advice Patrick.

Paul

 

[crayfellow]

Cool. I have a PL6 and this 1200W power supply. I am an EE and work with electronics for a living, so I have lots of opportunities to tinker. Having an extra PS with adjustable voltage is always useful for me, I've already used it for a purpose other than powering the charger

And, like you, it's important to quantify the value of your time and where you want to 'invest' that value vs. the occasional ready-made solution. I've already mentioned this, but so many people forget to factor that in when having these equipment vs. DIY conversations. Especially if you have kids, time is incredibly valuable!

 

[Jussi]

Hey Patrick. A couple more questions for you.

1. When you're deciding to connect power supplies to be able to utilize the power of the charger how do you know if you want to connect them in parallel or series. As I understand it, parallel would increase the amperage and series the voltage. I would think amperage would be more important because it would allow you to charge higher capacity batteries, no?

2. I'm a little confuse on what the limiting factor on how many batteries you can charge? Is it the power or current? For example the PL6. If I use only a single 12V 47A power supply it will supply the full 40A but only about half of the power capacity. Would I still be able to charge say 8 - 5000mah batteries (assuming I used a balance board)?

 

[crayfellow]

1. When you're deciding to connect power supplies to be able to utilize the power of the charger how do you know if you want to connect them in parallel or series. As I understand it, parallel would increase the amperage and series the voltage. I would think amperage would be more important because it would allow you to charge higher capacity batteries, no?

You can't get "more current" by changing the way you wire things. A given supply will be rated for a given current. People wire batteries in parallel on big copters to get more capacity for the same voltage; think of it as a bigger gas tank, not a bigger pipe.

What you are wanting to increase with a supply to a charger is POWER, which is (CURRENT x VOLTAGE). So even though there is not "more current" when you connect your supplies in series, there is more power. Think of it this way, if you have (CURRENT x VOLTAGE) with one supply, if you wire two identical supplies in series you end up with (CURRENT x VOLTAGE x 2).

It is that power that will help you charge higher capacity batteries, and/or more batteries in parallel, and/or charge faster. How much depends on current you have at your disposal given the (input power / the voltage of the batteries you are charging).

2. I'm a little confuse on what the limiting factor on how many batteries you can charge? Is it the power or current? For example the PL6. If I use only a single 12V 47A power supply it will supply the full 40A but only about half of the power capacity. Would I still be able to charge say 8 - 5000mah batteries (assuming I used a balance board)?

It is the power, which gives you the available current which will differ depending on the voltage of the batteries.

12V x 47A gives you 564W, assuming the supply is 100% efficient AND the charger takes no power to run. In reality, the supply is not 100% efficient (some is lost in the form of heat) and the charger takes some power to run. But for the sake of discussion we will use the 564W number.

A 6S battery has a voltage of 25.2V (the voltage depends on the cell count. To find the voltage of a given LiPo just multiply 4.2V x the cell count). So if we have 564W, our max current will be 564W / 25.2V or around 22A. In order to charge a 5000mAh battery at 1C (think of the C rating for charge as the "speed" of the charge) you need 5A. So you could theoretically charge 4 of these in parallel at 1C and be under your 22A budget (or 8 at 2.5A each, just means it would take twice as long to charge all 8. might as well charge 4 at a time at 1C at that point, and make sure you only ever parallel charge batteries at a similar level of discharge).

Does that make sense? You need to consider the input power, without thinking about input CURRENT or the max current of the charger, since those are just ratings for how much all the internal components and wiring can handle. Take the input power (max current of the power supply times the input voltage), then divide that by the battery voltage to figure out how much current you can "afford". That gives you a budget to work with to figure out how many batteries you can charge in parallel, and at what speed.

 

[Jussi]

@Jussi what did you end up deciding on?

So I ended up getting a PL8 and a 24V 40A power supply. In the manual, when setting the power management, it says to set the voltage cutoff at 50% of PS voltage. I understand the need for the safety buffer but That seems pretty excessive and would drastically limit the power output of the charger. Not a problem now since I only have 2 batteries but one of the main reasons I got the PL8 was its high power rating. Is 50% too low or is that a normal safety factor.

From manual...

1. Cutoff voltage should be about 50% lower than the power sup- ply’s nominal output voltage. Example: For a 24V power supply, set the cutoff volt- age to 12V. To protect your power supply, charging stops if the power supply’s voltage drops below the value you set here.)
   

 

[Jussi]

On a side note. I recently received a lipo battery from a friend who is getting out of the rc car hobby. It's 3s 5000mah and slightly puffy. Now I know if it's really big it's no good and dangerous. But what about if it's only slightly puffy.

 

[Jussi]

, and make sure you only ever parallel charge batteries at a similar level of discharge

Can you explain why? On the mpa board it only warns not to charge batteries of differnt cell counts.

 

[Jussi]

Any experience in using deep cycle batteries for on the field charging? Is the PL8 a good option for charging the battery?

 

[crayfellow]

So I ended up getting a PL8 and a 24V 40A power supply. In the manual, when setting the power management, it says to set the voltage cutoff at 50% of PS voltage. I understand the need for the safety buffer but That seems pretty excessive and would drastically limit the power output of the charger. Not a problem now since I only have 2 batteries but one of the main reasons I got the PL8 was its high power rating. Is 50% too low or is that a normal safety factor.

From manual...

1. Cutoff voltage should be about 50% lower than the power sup- ply’s nominal output voltage. Example: For a 24V power supply, set the cutoff volt- age to 12V. To protect your power supply, charging stops if the power supply’s voltage drops below the value you set here.)

that simply means that if you have your PS set to 24V, you tell the charger to stop charging if the input voltage sags to 12V. So at 24V your charger will behave 100% normally, as expected, but it will monitor the input voltage in case it sags in the event that your power supply is failing for some reason, as a safety measure.

So don't think of it as a limiting factor, but rather a way the charger can prevent a bad situation from getting worse if the power supply, wiring, or something else causes the input voltage to sag by more than 50% (from 24V to 12V, in your example). It is normal for there to be a little bit of variance so you may see your input voltage in the 23.x-24.x range.

 

[crayfellow]

On a side note. I recently received a lipo battery from a friend who is getting out of the rc car hobby. It's 3s 5000mah and slightly puffy. Now I know if it's really big it's no good and dangerous. But what about if it's only slightly puffy.

Your charger can calculate "Internal Resistance" for a battery. This is like the "health" of the battery, per cell. So before you use it, check the IR for each cell, and stay with it when you charge it to feel it and see if it gets more than lukewarm. If it does, its internal resistance is creating heat and this can lead to even more heat under discharge (or even when it's just sitting there, if you let it sit when discharged too low, or fully charged) which can lead to combustion.