Today’s Dumb Battery Question

[Greg Elmassian]

Actually it is fine to leave lithium cells "charging" because a lithium charger shuts OFF when the battery is fully charged.

There is NO "trickle" or "float" charge used in lithiums. You must resist applying what you "know" about nicads and nimih batteries to lithiums, most is not applicable at all.

I'm an engineer, and it is still unbelievable that many of my associates who have degrees in electronics still have bad information about batteries.

I would encourage every person that has batteries to have at least one good "multi type/cell" charger... this is not "smart", but usually settable for at least battery chemistry, number of cells and max current.

Greg

 

[FatherMcD]

I would encourage every person that has batteries to have at least one good "multi type/cell" charger... this is not "smart", but usually settable for at least battery chemistry, number of cells and max current.

Greg, would you share your rational behind this? Are you suggesting there is a problem with smart chargers or just that it is good to have a settable charger when needed or...? I have a settable charger, a couple of smart chargers and a dedicated 14.8v Li-Ion charger (a "less-smart" charger?). I generally buy what the battery manufacturer recommends, except for the settable.

 

[Greg Elmassian]

I have so many rechargeable devices that it is convenient to have a charger that can be set for different chemistries and voltages.

There used to be "auto sensing" units that could automatically detect the number of cells and chemistries, but it's very tricky and could also get someone sued.

Let's forget the term "smart", it really sort of means nothing, it was meant to describe a charger that could sense the charge state of the battery and adjust charging current appropriately,

What I am suggesting is smart in that manner but adjustable also.

The opposite of "dedicated" to a fixed voltage and/or fixed chemistry and/or current.

Greg

 

[dunnyrail]

While Lithium cells are predominantly safe there can be cases where ‘run away’ due to incorrect charging or poor quality chargers being used can cause disastrous fires. Knowing the risks is worthwhile knowledge and the following vid shows what can go wrong.

I have posted about this before with other vids and when you look at YT there are many vids, a lot done by numpties that smash them with hammers etc asking for a dramatic show. This one below appears to be somewhat informative. Showing that cells can be bought that are not fit for purpose.

 

[ACD134]

So 10 AAA will give you 8 watt hours at 12 volts. go a bit slower and you can get you 5.6 watt hours with 7 batteries, i.e. you can run about an hour with 7 batteries.

Greg. Are you sure about your maths? 10 AAA (in series) will still give you 0.8 Watt Hours at 12V. You would need to put them in parallel to give you 8.0 Watt Hours but at 1.2V, this can bring oth

 

[Greg Elmassian]

watt hours are watt hours... irrespective of voltage.... watts are energy...

the volts and amps will change, but watts are energy/power.... series or parallel, it is still additive.

this is the reason you use energy or watts, in the USA we have 110v and 220v in our houses, but we are charged in watt-hours...

notice electric vehicles are rated in watt hours.. not voltage and current or series or parallel (even though that is part of the construction)

Bottom line is each battery is 0.8 watt hours, how you combine them is your business.

Notice no indication of run time or current drawn. 12v will deplete the battery pack 10 times faster than 1.2v (given same current draw) but don't go there, you will get even more confused...

watts is irrespective of current and voltage, it is the multiplication of the two.

Greg

 

[Henri]

Trying to get my old electronic knowledge back up to where it used to be...

I think you all are wrong regarding how to calculate a reasonable capacity for the battery pack for a loco.
The error is that a battery (milli)Ah rating is not the same as (milli)Watts per hour!

(Milli) Watts per hour is a different factor and most of the time is not specified by the battery manufacturer, so we don't know it. (you can cheat by multiplying the voltage over the (m)Ah rating, that is the power the battery will give for ONE hour, but it is a useless number for our calculations)

Following example is based upon 1,2 Volt, 1500 mAh batteries.

Measurements
First you need to determine the voltage and amps used while running your train (locomotive including wagons!). Set a speed that satisfies you. Then measure the voltage and amps.

(max) Speed
The voltage is used to determine the number of batteries needed to achieve the voltage require to run the loco nicely.
In this example you can use 10 batteries in series (making 12 V). The loco will run nicely.
(Phil's correct addition): voltage will drop during running time so the train will run slower after 'a while'. Using more batteries than required will postpone this moment, but

However, this will not normally be a problem, as we do not tend to run at full-speed, or constantly, over a normal running session.

Duration/running time
The Amp ratings are used to calculate the running time for a battery pack / loco combination.
With the 1500 mAh rating (which is 1,5 Ah), the loco will run: 1,5 Amp (batteries) / 0,25 Amp (loco) for six Hours.


Increasing running time
Putting batteries parallel will double the mAh value.
In given example, if you put parralel two sets of 10 batteries that are in series, the mAh will double to 3000 mAh and your loco will run for 12 Hours.


Conclusion
Please correct me if I'm wrong, but this is how I remember my electronic lessons...

 

[PhilP]

Hello Henri,
Yes, you are correct..
But there are a couple of things to consider:
Yes, battery voltage sets the top-speed for your model.
If you run for a long period; The battery voltage will drop off, as you begin to fully drain the battery. - Your model will still run, but will get slower.

However, this will not normally be a problem, as we do not tend to run at full-speed, or constantly, over a normal running session.

PhilP

 

[Henri]

Hi Phil, yes I should have added that as that is indeed the case!

 

[dunnyrail]

Interesting info from Henri and Phil.

Other variables also can effect run time, length of train, roll-ability of each vehicle, weight of each vehicle, how much drag is there in the locomotive ie drive train, valve gear, left in pickups. Gradients on the line also curves. All can make admittedly small differences to run time. For my part with between 8 and 12 AA/AAA batteries of varying mAH ratings I can get between 6-8 hours running or at least time with the thing turned on. Probably not the same as someone who runs a train round in circles continuously as I tend to do shunting, waiting for other trains to pass etc thus possibly not calling on battery power quite so much.

I think sometimes we can over think these things. If you do a conversion and not happy with the run time, adding more batteries would likely solve the problem. But this is not an option for all locomotives.

A good rule in our scale may be to put in as many as possible to fit but probably not much more than 14 NiMh batteries as at this level you may be getting over the top with Rx and Speed Controller (some may take more check voltage on instructions) plus your loco on a full charge at full speed is likely to set off like a scalded cat.

 

[PhilP]

A good rule in our scale may be to put in as many as possible to fit but probably not much more than 14 NiMh batteries as at this level you may be getting over the top with Rx and Speed Controller (some may take more check voltage on instructions) plus your loco on a full charge at full speed is likely to set off like a scalded cat.

The number of cells (hence maximum voltage) will give you the top-speed for your loco..
The (physical) size of the cells, will determine the amount of charge (energy) that can be stored. - Hence the run-time.

Think of the batteries as holding the 'fuel'. - Bigger batteries, more fuel, longer range.

To many volts, will make it difficult to control your loco.
If you exceed the design voltage of your motor, you will eventually have problems with over-heating, and possible damage.

PhilP

 

[Greg Elmassian]

Henri:

I do no see where anyone said AH = watt hours.

If you think It was me, you need to read more carefully.

The reason I introduced watts is that the amps of the loco will vary with load and voltage.

Yes, you can use amps only, but then you have to specify the voltage applied to the motor also at that current draw.

Guess what, voltage times current is watts.... so you are INDEED using watts in your calculations.

V=IR
P=VI or I**2 R

Learned in high school too.

Greg

 

[Paul M]

I**2 R

I assume that means "Amps squared X Resistance"