Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[ChrisH1981]

Im trying to choose between lipo or nimh in my latest project. I have a brushed motor set up, motors are drawing high amps. Dont want to change motor and speed controller set up. Currently i am using NIMHs as power supply, but they are getting hot when in use. I understand this is the battery not up to the job of suppling the amps that are demanded by the motors. So what is this solution? Im not very knowledgeable on lipo technology, is there a lipo battery that is able to supply a large amount of amps without running out of juice quickly? Is this determined as the 'C' rating?! Sorry if my questions sound basic, but im a bit prehistoric regards battery technology!

[Steve Dean]

Let's start with the easy stuff first. You say the motors are pulling a lot of current. Do you know how much ..... have you measured the current.Also you do not state the voltage of the NIMH pack(s) and their current rating. As an example 7.2 volts / 5000 mAh.
If you can supply some answers to these questions then we can give you some proper answers.
To get you started on understanding LIPO batteries ........ the first thing to understand is that they have multiple cells just like most other types of battery. Each cell is nominally 3.7 volts therefore a LIPO described as being 2S has two cells and therefore is a nominal 7.4 volts, 3S equals 3 cells and therefore 11.1 volts, etc, etcThe 'C' rating is the Capacity rating of the pack and is a multiplier. To give you an example that is easy to understand ......... if you had a pack rated at 5,000 mAh (5 Amps) with a 'C' rating of 10, it has the potential to deliver 50 Amps (5 x 10), BUT it can only do this for a tenth of the time. Therefore instead of being able to deliver 5 Amps for 1 Hour, it will in theory deliver 50 Amps but only for 6 minutes.
I'll leave you to get your head round the above and hopefully answer the questions I posed and then we'll go from there.
Very pleased to helpSteve Dean

[SteamboatPhil]

Well that helped me Steve (let a steam man loose with batteries.. ). See you in November 

[ChrisH1981]

Hi Steve, thanks for your reply. As usual I forgot to go into detail! So the nimhs are 7.2v 3300 mah. I haven't measured the amp draw of the motor, im hoping to do that this weekend once ive made up some leads to connect to the motor and multi tester.


Thanks for the LIPO explanation, very interesting!


Im now thinking how this compares to nimh batteries....can they supply amps in the same way as lipo's?

[Steve Dean]

Hi ChrisH1981,
I note you are using a 3,300 mAh NIMH pack. This mean the pack can deliver 3.3 Amps for 1 Hour. If you were to pull twice this current from the pack (i.e 6.6 Amps) then it would only last approx 30 minutes.These packs are also available with capacities of 3700, 4300, 4600 and 5000 mAh.As an example a 5000 mAh pack (5 Amp rating) could deliver 5 Amps for 1 hour and therefor if you were to pull 20 Amps from it continuously then it would only last approx 15 minutes (and potentially get rather warm).In reality one doesn't tend to run the boat flat out all the time so you get a slightly longer period of operation.It would be interesting to know what motor(s) you are using. I can then tell you the typical current the motor the motor will pull (this of course is dependent on a number of addition factors, prop size & pitch, weight of the model, etc, etc).Cheers,Steve

[Subculture]

Something worth bearing in mind is that NiMh's rarely give their full capacity or even close to it when drawing high currents.

Lithium batteries on the other hand are capable of delivering very high currents and maintaining their specced capacity.

[chas]

After many years of using ni cads then nimh batteries, my first lipo battery was a real eye opener. The difference in the ability to deliver power, and keep delivering at full voltage was a revalation. It went in an RTTL, drawing around 12 amps, not a huge amount these days, but still plenty. The light weight helps too, it's made the model a real pleasure to sail.
   I would encourage anyone who needs plenty of amps in a model to use lipo batteries, with one provision. You have to be prepared to charge properly and safely, and always store them at the correct storage voltage. All that needs is a decent charger, and a little self discipline.
  Getting used to these batteries encouraged me to build larger fast scale models, like my latest Gar Wood special, I'd never have bothered if a decent power source hadn't been available. Try them out, it's well worth it..
Charles.

[Fastelectrics]

You may find that the motor is trying to turn a prop that is too large and may not be operating efficiently. Reducing the prop diameter will reduce the amp draw and should provide more run time. How long are your batteries lasting with your current set up?

[ChrisH1981]

Many thanks all of you for turning theis into a very interesting conversation! I am starting to get my head around this now!. Ok so the model in question is a 1:12th scale Brede lifeboat, 33 inches long, 35mm 4 blade props. The model was second hand and came installed were 2 '700' series motors, no markings on the can......cant find reference to these exactly on the internet buy dimensionally they are the same as 775 motors. No idea as to the amp draw free running or in the water, mainly because i dont want to mess about with the wiring to add an amp meter into the circuit (would do this as last resort)

@steve dean thank you for your time in explaining!

So i am now looking at LIPO battery technolgy and fully understand how to care for them! I am now thinking something like the overlander supersport 2s 5000 mah would be a good choice, but what C rating would give me an nice balance of duration and performance?

How many amps could the 2 775 motors be drawing in the water? Props have a fair pitch on the,m but the motors barely get warm in the bath test I have done, just the batteries!

For further info, there are two 30amp speed controllers in this set up, and 2 20amp fuses.

Im thinking the motors are probably pulling 15 amps in the water???

[ChrisH1981]

You may find that the motor is trying to turn a prop that is too large and may not be operating efficiently. Reducing the prop diameter will reduce the amp draw and should provide more run time. How long are your batteries lasting with your current set up?

Roughly 10-15 mins at most :)

[Steve Dean]

Hi Chris, the 700 series motors were available in a variety of ratings voltage wise (8.4, 9.6 or 12 Volt) with a current draw ranging between circa 12 - 18 Amps. They fact that your 20 Amp fuses have not blown clearly is proof of this. Your stated run time of circa 10 - 15 minutes also fits in with this.If you used a 5000 mAh pack then the run time at 15 Amps would be around 20 minutes. Although in reality it would be longer because it is unlikely you will be at full throttle all the time. If you want to use a LIPO pack (assuming 2S i.e. 7.4V) then using a pack of around a 35C rating would be a reasonable choice. Choose one with the highest mAh figure you can get. They are available typically with 5,000, 5400 & 6000 mAh capacities. You can get them higher ....... up to 10,000+ (with an appropriate price!!!).Depending on what speed controllers you are using, you could go to a 3S LIPO (i.e 11.1 Volt), however for safety the motor needs to be the 12 Volt version.Any further questions please feel free to ask. Steve.

[ChrisH1981]

Thanks agian steve...and everyone else! I have a great understanding now and can start to make an informed choice on battery!
Im going to stick with 2s. Now another thing to mention is the current setup is 2 7.2 nimhs wired in series to give 14.4v (3300mah).

1.  Will this damage the motors seeing as their voltage is probably a max of 12v and
2.  Can lipos be wired in series without cause for concern?

[justboatonic]

If you start using Lipo batteries, make certain your charger will charge them and that it is set to charge Lipos not Nimh \ Nicads or even lead acid batteries.
Also, Id strongly recommend you either charge outside on a non combustible surface or, charge the Lipo in a fire resistant bag and sit close by so you can see any potential charging issue. Yes, it's like watching paint dry and yes, if you take care you will not have any issues. Some people do and unfortunately, the consequences can be quite devasting.
Id also point out Lipos have a charge rate known as 'C' and a discharge rate. The discharge rate is also expressed as C but usually in the format 25C to 50C etc. This is different to the charge rate 'C' and is an indication of how quickly, in bursts, the pack can give up its power. Never confuse the two.

Charge rate 'C' is expressed, generally, as multiples of the pack's capacity. So if a pack is 3500mah, its best charge rate is 1C = 3500mah. If the pack is 5000mah, 1C = 5000mah and so on.

Lipos can be charged at multiple 'C' rate but Id recommend you stick to 1C whatever your pack's capacity is to prolong the life of your cells.

Never charge your pack at '25C to 30C.' You will have a fire if you do!

[Steve Dean]

Hi Chris,I'll answer your 2 questions. Q1 = As you have the two 7.2 Volt packs in series giving 14.4 Volts nominally (although slightly higher when fully charged), you are over running the motors a bit. However the voltage will drop off quite quickly, so other than the motors getting a bit warm all will be OK. I'm sure you understand that if you ran the motors over voltage at high current for an extended period of time then both the motors AND the speed controllers (plus the batteries) could get more than just warm!!!Q1 = Yes, you can connect LIPO's in series (like any battery) to increase the voltage. However, as you may understand, the current rating stays the same. When doing this I recommend that you use decent size gold connectors that can't be plugged together incorrectly.The advice given by the previous person regarding LIPO's is very sound. I would add that using a charger whereby you can see the individual cell voltages is highly recommended. This is referred to as Balanced Charging. The charger will monitor the cell voltages and attempt to charge the individual cells to the same level. As the pack ages this become more difficult for the charger to achieve and it is not unusual to eventually have a cell in a pack which starts to degrade.Cell charging rate is area that can lead to hot debate (pun intended). Whilst many packs are specified to have a CHARGING 'C' rate higher than the fundamental capacity of the pack, i.e 5000 mAh (5 Amps), doing this will cause the pack to get hot when charging and over time will cause the pack to deteriorate.I'm trying not to baffle you with too much information, so I hope you're hanging in there!Steve.