4.8v or 6v?

[Pilot Ben 28]

 

Hi Everyone,

 

I currently have a 4.8v 1700mah rx pack in my mini hype. I had three flights and checked the battery and it was at 22%. I am now going to get a 2500mah pack, but do I get 4.8v or 6v? I am undecided because I have 4 very big metal geared servos (HS645MG) and wondered if 6v would make them happier and make the pack last longer...???

 

Thanks,

B

 

[Lofty 5]

 

The higher the Volts the less Amps! So a 5cell will last longer than a 4 cell battery! But having said that the more you throw the plane about, the less the battery will last anyway.

 

Also running a 5 cell over a 4 cell with make your servos move faster and have more pulling / pushing power!

 

Make sure that the servos you are using can run on 6V+............most can these days and they will show there power / speed for 4 / 5 cell battery's in the info part of the box!

 

For Spekrum RX, they have said for the last 6+ years......you should use a 5 cell battery all the time! "or Li-fe / A123"

 

I would never use any battery under 2000mAh these days in a standard RC plane!

 

3 flight's slime a bit low even for a hype not doing hardcore 3D! Maybe try cycling that battery to wake it up!

 

Have a look at the 2 cell Li-Fe / A123 battery's they are 6.6volt, don't weight or take up as much room as NiCd or NiMH........but you will need a charger that can use them!

Li-Fe / A123 do cost more and only come in a few sizes atm!

 

I am not sure where you got your battery from! But try http://www.vapextech.co.uk/ , been using these for years.........had hundreds only had 1 or 2 go bad!

 

If I think of anything else, I will post later!

 

[Martin 0]

 

We've had this and related battery discussions several times over the years on the forum. For example:

viewtopic.php?f=22&t=3442

 

Firstly, I would be very sceptical of any device that claims to check a nickel-chemistry battery and say how much charge it has left as a percentage. Nickel-chemistry cells (such as NiMH) don't work that way. Have a look at the discharge graphs on this page:

http://main.panasonic-eneloop.eu/en/cap ... t-capacity

Note how the voltage starts at about 1.3-1.4V/cell, and rapidly drops over the first 10% discharge to 1.2-1.3V/cell, but then the graph flattens out and the voltage doesn't change very much until it starts to drop more rapidly around maybe 70% discharged. Note also how the exact voltages are actually different between different cell types (in this case, Eneloop and some non-LSD NiMH). How can a simple voltage-based device possibly give you two significant figures of accuracy of capacity remaining? The difference between Eneloop and non-Eneloop on this graph is more than that representing over 50% of capacity!

 

The device I lent you at the field last week was simply a yes/no battery tester that first checks the battery with a small load, then when you press the button applies an additional load of a couple of amps or so while still measuring voltage. It's a very useful device if you use nickel batteries, but it's intended to catch cases towards the right-hand side of the discharge graph - if it shows amber or red then your battery is getting into the part where voltage drops off quickly, and you should recharge before flying.

 

The only way to really know how much capacity is left in a nickel-capacity battery is to discharge it at a controlled rate (say 0.5A or 1A) to a known end voltage (say 1V/cell or 0.9V/cell). Try using your tester then doing this - you may be surprised. Note that you can't draw many (if any) conclusions from the amount of charge you put back into the battery - the cycle efficiency of nickel chemistries is not great (so you have to put quite a lot more in than you can get out), and the end-of-charge is not very precise (so you may keep charging the battery for an hour or more after it's actually fully charged).

 

Generally, the reason for going from 4 cells (nominal 4.8V) to 5 cells (nominal 6V) is not to get more time between recharges. The bigger advantages are:

 

• more speed and torque from the servos. For example, your HS645MG improve speed from 0.24s to 0.20s and torque from 7.7 kgf.cm to 9.6 kgf.cm between 4.8V and 6.0V. Depending on how you look at it, you're either getting better performance without having to pay for a more expensive servo, or you're getting the full performance that you paid for out of your servos.

• a better safety margin if the battery is low. If battery voltage drops below a certain level you can have a thing called a "receiver brownout", which means the voltage at the receiver is too low for the electronics in the receiver to keep running. Quite often, this causes the receiver to re-initialise as though you've just switched on. Exactly what voltage this happens at, and how long it takes the receiver to come back, vary greatly between types - I seem to recall that when Spektrum gear first came out one of the big criticisms was that it would brown out quite easily and take several seconds to come back, during which time all servos would hold their existing positions (I think this is better now).

 

 

The thread I linked to at the top also includes some discussion about whether or not a 6V pack would indeed last longer than a 4.8V pack. To summarise the discussion from memory: the higher voltage means that it will push more current through your servos, so you'll be draining the battery faster, however this may be compensated for to a certain extent as the servo moves faster at the higher voltage so the servo should be moving for less time. I don't think we ever came to a conclusion about which would get the longer actual lifetime, and it would probably depend on a lot of other real-world factors. It is likely that a 5-cell pack will last less long than a 4-cell pack.

 

One thing I would definitely say though is to look at good quality low-self-discharge NiMHs (Eneloop are good but expensive, and Vapex Instant seem OK), rather than the cheaper high-capacity ones. You mention getting a 2500mAh pack - if this is a non-LSD NiMH then I would not personally do this as the non-LSD NiMHs lose their charge very quickly in storage - you need to charge them on the day you're going to use them. There are "Eneloop XX" available (albeit expensive) which have a high capacity and better self-discharge, but I have not personally used these.

 

Another option, which I have used successfully, is lithium-iron-phosphate (commonly called LiFe or LiFePO4; "A123" is a brand name for the technology). This has a nominal voltage of 6.6V and a maximum voltage of 7.2V - a 5-cell NiMH pack has a nominal voltage of 6.0V (though an LSD pack is probably 6.5V) and a maximum voltage around 7.2V (the 1.4V on the graph times 5 cells). I have a 2S LiFe pack in my WOT 4, and one in my Slipstream glider, and they work very well. Two great advantages of these is that they charge much more quickly that nickel cells - I charge mine at 1C like a LiPo but some can go higher, and that you can top them up very quickly if they're already charged - if I can't remember whether I charged the plane or not I just put it on the charger, and if it was already charged the charge will terminate within a minute or two with no more than a few mAh added. (Their self-discharge is also very low, so they'll still be fully charged even if you charged them a while ago.) If your servos can cope with 5 fully-charged LSD NiMH cells, then they should be able to cope with these.

 

[Pilot Ben 28]

 

Thanks guys, I'm now going to get a 2000mah 5cell, as the 2500mah 5cell won't fit

 

See you soon,

 

B

 

[Martin 0]

 

Make sure to get a low self-discharge battery though!

 

Also, you might want to try several discharge/charge cycles on your existing battery - both to see if the capacity improves, and also to see how well it actually performs.

 

[Lofty 5]

 

I've also seen some AC/DC chargers not doing there job

 

E.g Showing / saying fully charged, when the battery is not!

 

But having said that, if it is only the battery in the mini hype, then it might just be a crapo battery! Do a few test etc etc!