Choosing Batteries

Posted Mon, 05/30/2011 - 14:34 by Brian

Holy <bleep>ing <bleep> is space tight in Nassau... I knew batteries were going to be tough but this is nuts... Last meet's discovery that Nassau basically needs to run at 12 VDC to get to speed was a major blow. It basically knocked out any possibility of using sealed lead acid batteries (SLAs).

To make matters worse the little micro pump has to run at 6 VDC or it will burn up... So I either need three 6 VDC packs or one 12 VDC and one 6 VDC... and then there is the space restrictions...

I started looking at building up battery packs. Unfortunately 1/2-D and Sub-C cells are only 1.2 VDC each... Either of these cell sizes would result in some very weird pack shapes...

LiPo is out because of safety concerns... So I started looking at LiFePO4 cells. I think I found a combination of packs while will fit... barely.

Propulsion and radio will run off of a rectangular pack of eight 18650 LiFePO5 cells. This pack runs at 12.8 VDC for 3000 mAh... This pack will just barely fit under the main CO2 regulator just behind the forward turret. This is not as much endurance as I hoped for but it is probably all I can get. So I might have to have a spare pack (ouch)

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I have a couple of other small hollows in the ship - one of the larger ones is aft of the CO2 bottle just forward of the aft turret. This space is much smaller but I can fit a pack of two 18650 LiFePO4 cells for 6.4 VDC at 1500 mAh. I think this is probably fine for running the pump a good long time. However, these packs are relatively inexpensive - so a spare will not be too hard on the wallet.

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I will have to run these packs by Drew as he has forgotten more about battery tech then I will ever learn. However, these packs will fit (barely) and should be enough to get me through a sortie or two before needing replacement.

 

UPDATE: WARNING ABOUT BATTERYSPACE.COM

The batteries I received from batteryspace.com proved unusable in packs. Their voltages were very inconsistent which made charging the packs impossible. Closer inspection showed that the cells were marked to show which ones were low in voltage and which ones were not. Unfortunately I didn't realize the problem or the significance of the marks until most of the cells were in packs. The batteries each individually charged fine - its just that the voltages varied a lot so they couldn't be charged in packs. You end up with packs with low capacity which can only be changed with a balance charger without overheating.

I took encouragement in the fact they had clearly identified the low-voltage cells before shipping them. So I placed an other with them and explained that the batteries were going in balanced packs. I said that I needed cells that could be consistently charged to the same voltage as each other. They contacted me and explained they could not promise that.

Be advised that their batteries vary in voltage +/- 1VDC from each other. If you want to use their cells in packs then you will have to order a lot of extra cells and hope to get the right number with the same voltages.

 

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Drew's picture

Re: Choosing Batteries

by Drew - 05/30/2011 - 15:16

LiFe batteries are normally 3.3v per cell. The main pack will be around 13.2v  and the RX pack will be around 6.6v.  Depending on how the packs come wired you should be able to charge them VERY aggressively between sorties if you want but that will require a power source and charger capable of pushing that kind of power.  People regularly charge LiFe packs at crazy rates such as 5C (or higher) or a rate equivalent to five times the capacity.  This results in a turn around time of under 15min.  For your larger pack, this would mean charging at 15A or around 220w input power to the charger.

 

For your servos, you'll need some kind of voltage regulator to step down the voltage of the receiver pack, so you could run everything off the main pack and just use a switching regulator and eliminate the receiver pack.

 

http://www.readyheli.com/Voltz-6250-iCharger--250W-Synchronous-Balancing-Charger_p_37876.html

That charger is one of the nicest on the market today that is fast, precise, reasonably priced and very capable.  It is essentially a rebranded "iCharger 106B+" made by Junsi.  I also have an extra dual channel charger that I could sell you that is also quite nice but a bit older.

Brian's picture

Re: Choosing Batteries

by Brian - 05/30/2011 - 20:15

The smaller pack is for the pump. The speed controller I am using provides 5 VDC power to the transmitter. So servos will not be a problem. I've already been running the radio off of the output from 12 VDC.

That Voltz charger looks nice but the box the charger comes in, the housing of the charger itself and many of the sites describing it, indicate it can only charge LiPO/LiFe packs with up to 6 cells. So it may not be a good match for the 8 cell pack I want to use...  The manufacturer's site indicates that the charger can be expanded to 8s if not using load balancing... but then I could also use a much cheaper charger too

 

Brian's picture

Re: Choosing Batteries

by Brian - 05/30/2011 - 20:27

Ok - something did not add up... so there are only 4 cells in series. (two sets in parallel). How can I tell what chargers will work with this pack?

What power supplies can I use for the Voltz? Another $70 for the charger is making this pricier than I am comfortable with...

Brian's picture

Re: Choosing Batteries

by Brian - 05/30/2011 - 20:49

Last question (tonight) I promise. Is there some reason not to go with the Hyperion EOS0606i which has both AC and DC input (removing need for separate power supply). It seems like a pretty popular charger so I figured there was some reason you didn't like the Hyperion chargers.

Drew's picture

Re: Choosing Batteries

by Drew - 05/30/2011 - 21:49

I'm silly.  I forgot the part about the pump.  Hyperion does make nice chargers...  I think I have 5 of them but 4 of them are 0610 DUOs which are 180w/channel and will run off up to 28vdc.  The 0606i AD is a decent charger but only 50w output power, so it would limit you to about 4A into the larger pack.  That would mean around 50min to charge the pack.  AC/DC chargers are always a compromise.  Many people use modified PC power supplies to power their chargers at home.

Brian's picture

Re: Choosing Batteries

by Brian - 05/31/2011 - 19:31

Drew has tried talking me into making my own packs... I have a few questions about pack composition:

What is needed to support cell balancing?

Most of the better chargers support cell balancing. From what limited reading I've done it sounds like this requires special wiring in the pack. Is that true? Is the connector standardized or does each charger brand have their own standard?

A better question is why do I care about cell balancing?

Are all A123 18650 cells the same?

I am seeing prices vary wildly from site to site. For example, this site (link) lists them at $2.18 each; However, Hobby King (link) sells them for $8.06 each. Don't get me wrong - there are a few red flags with the first site (telephone number for example). However, it also sounds like there are a lot of fake A123 cells as well as 'factory seconds' out there. That seems to render the brand pretty meaningless if you can't know you are getting the real deal

I suppose a closely related question is why do I care if they are genuine A123 cells? It sounds like the A123 cells sold by Hobby King (link) frequently have problems. The off-brand 18650 cells at Battery Space (link) seem to have similar specs and are about half the price typically listed for A123 cells.

Brian's picture

Re: Choosing Batteries

by Brian - 05/31/2011 - 20:00

I found what looks like a good page for describing balancing and why it is important (link). It also describes the different types of balance taps.

Drew's picture

Re: Choosing Batteries

by Drew - 05/31/2011 - 21:50

I skimmed the page and it does seem to have some good info. Most chargers today have internal balancers and use balance tap adapter boards.  Allegedly, some chargers today will not charge a Lithium based pack if no balance connection is detected.  IIRC, balancing isn't as critical with LiFe and especially so when used at lower discharge rates.  All the same, I prefer to have balance taps on my packs so I can at least monitor the individual cell voltages. 

 

BTW, I was going to give you the A123 cells that I have because I have no intention of building any packs with them.

Brian's picture

Re: Choosing Batteries

by Brian - 06/01/2011 - 06:08

Thanks. I don't mind buying them but right now I am trying to convince myself that (a) I can build good quality packs and (b) I can do this for a somewhat reasonable budget. Maybe I will grab your cells and see how soldering them goes. I assume the ends are aluminum... so do I need zinc flux & solder? do you know if there is somewhere around here to get battery tabs and battery soldering supplies?

Brian's picture

Re: Choosing Batteries

by Brian - 05/31/2011 - 21:43

I also found a good page (link) describing how to wire packs up including the balancing taps. You will need to enable JavaScript for the page to work correctly. There was another page (link) that looked good at a glance but I've not really had time to read through it carefully.

I also found a listing with a top rated eBay seller (link) for A123 18650 cells - and they will weld up packs or just tabs. so building packs is starting to seem reasonable. Plus if this seller is legit then the packs should be pretty cheap to make.

 

Brian's picture

Re: Choosing Batteries

by Brian - 06/02/2011 - 19:39

Tonight I played around with some wood mock-ups of LiFePO4 cells to see what would fit... it didn't go as expected (read more here)