Battery Power

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Questions and Answers

How do you make a guitar amplifier battery powered?

I have a 11.2 Watt First Act guitar amp and I was wondering if it's possible to modify it from using a 14V power adapter to making it battery powered?

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A battery is a power tank. The longer you want to jam, the more batteries you need. At 11.2 watts, you need 2.33ah per hour of battery bank (11.2 watts/12volts * .8 for inefficiencies * .5 battery DOD) = 2.3a.

Long story short, this 33 ah battery is 14 hours of jamming, weighs 33 pounds, and you can get one here under $120 delivered. UL Listed, made in the USA, and designed for use indoors, it won't spill acid.

Get a little box for it here and you have a really nice setup that folks won't complain about.

Next you need to regulate the voltage coming from the battery to the amp. To get a 12 volt battery to run a 14.4 volt load, you need a transformer. This one goes from 12-15 volts at 50 watts, the voltage you need with more power than you can use for roughly twenty bucks. Tell them BD Batteries referred you.

So you are completely mobile for $150

Battery chargers are $30 – $400. Any of these will do.

Understanding DC power (batteries)?

As a vivid photographer, hiker, and random hobby enthusiast I use a lot of battery powered devices. I have different battery types (alkaline, Li-ion, lithium) depending on the application. I know what batteries work well with what device, but I don’t know why exactly.

I know that there are several components of DC current and storing its power. I’m having a tough time trying to get my brain around the workings of it. Please read all the examples and then reply as a whole, as it would help understand the functions and differences.

For example:
Camera Flash: When I attach my flash to my camera I always use Ni-MH (nickel–metal hydride). Ni-MH batteries have a much quicker recycle time/charge time between flashes. Alkaline takes twice the time to cycle and can only take half the photos compared to Ni-MH before they are dead.

Flashlight: When hiking and camping I use high end flashlights that use LEDs to illuminate. Alkaline batteries are brighter than Ni-MH and have a longer run time. I understand this is due to their higher voltage than NiMH.

Why does alkaline in this circumstance last longer than alkaline.

Here are the battery facts (although I may not fully understand them)
-Ni-MH have around 60% the mAh compared to Alkaline.
-New alkaline batteries sit at 1.5 volts new, while NiMH sit at 1.25 volts. Alkaline battery voltage drops as they are used. NiMH stay at 1.25 volts until they are “empty”, there is no battery fade.
-Ni-MH has an internal resistance 4X lower than alkaline.
-Ni-MH lose their power much faster than alkaline. NiMH lose around 1% of power per day, while Alkaline lose 1% every 30.

It appears high-power device draining equipment favors NiMH even though they are 60% of the MaH rating. Why does a device like this favor NiMH when alkaline has higher voltage, MaH, and less battery fade?

Posted by
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There are several important battery specifications. Usually you can find data sheets (pdf files) on the internet for your specific battery type (model number and brand).

Preliminary explanations:
Voltage is the electrical pressure that drives electrical current through an electrical load (resistance). Power is voltage * current, in watts. Energy is power * time, in watt-seconds (joules) or watt-hours. Ohms law V = IR. Internal resistance is what causes voltage drop from the open circuit (no load) voltage. Look these up for deeper explanations.

Storage capacity is the total electrical energy that can be extracted, often called "a charge". This is measured in joules, which are watts * seconds, and is often quoted as ampere hours (Ah) which are derived from joules through the battery voltage and time in hours instead of seconds. This is because watts are volts * amps. With an "ideal" battery this would be enough specification on its own, with some secondary specifications like voltage and cost and weight ratios. In practice we might need to draw particular currents from the battery to suit the device it is powering. This changes the story. In some cases the Ah rating is changed by the current drawn, and the temperature. Capacity is usually specified at some standard rate like over 10 or 20 hours and standard temperature like 25°C. Non-rechargeable batteries tend to have the highest storage capacity, for example D size cells, alkaline maybe 14Ah, Nicad 4Ah, NiMH 8Ah.

Another specification may be even more important, current capacity. This is the number of amps that the battery can deliver to the load. It is used with car batteries to specifiy the "cold cranking current". It is used to show the maximum current sustainable for a short time without damage and withing some voltage range. It is related to the battery "equivalent internal resistance" as well as its physical size and construction. Rechargeable batteries usually have high current capacities compared to say alkaline non-rechargeable batteries.

Thus there are two capacities, the Ah capacity and the current capacity. Both are specified in the data sheets for particular conditions. These conditions tend to be chosen as the "best case". In practice the storage capacity (Ah) in particular may less, because of actual load and temperature.

The voltage per cell is a separate issue, related to the chemistry. A cell with high voltage like lithium types tends to mean higher energy density (per weight) as less cells are needed for a given storage capacity.

What you say above in comparing different cells is basically true, but the shelf life of NiMH may be extended in some types, as there is a trade off between capacity and shelf life. To know the true story find the data sheet.

The practical storage capacity might relate to the actual voltage range that the equipment needs. A 12V lead acid battery is usually considered charged at almost 14V and discharged at around 10V. If your equipment only works down to 11.5V, clearly the specified capacity will not be relevant. This applies to all battery types. The "discharge curve" is important, how it aligns with the actual equipment used. Also the battery "end of discharge" voltage may be set higher to extend the battery life (number of charges it can receive). This reduces the capacity from that specified.

The first link below might help with using batteries. See the discharge plot. You will see discharge rates mentioned as C/5 etc. The C means capacity. An example of C may be a battery specified as 2Ah at the 10h rate. Thus the current is 2Ah/10h = 0.2A for 10h. This is a C/10 discharge rate (0.2A for this example). A 0.5C rate is 0.5 times the current implied by the Ah rate (so 1A for this battery). The time is 2h (if all things are equal, which they are not). You may see Ah or charge times specified for 0.1C, 0.2C, 0.5C rates etc. This is the storage capacity with a given discharge current.

The standard charge rate is usually C/10, so for this battery, 0.2A. Note this is usually 14 or 15 hours for rechargeable batteries, at the 10h rate (C/10).

The second link is a list of spec sheets (some are only MSDS sheets I noticed).
The third link is an example data sheet for a NiMH cell.

Dell Laptop. Battery Power and Ac?

If I use the battery on my Del;l Inspiron 6400 the screen is darker even although it says at the moment battery is 97% ok. When I switch to AC it comes back ok.Have also noticed when I leave the computer which should go into hibernation mode after a certain time (cant remember how long)it does go off but when I then switch on again it says its shutting down and have to then wait until it goes through all the procedure and then have to start up again. Am a senior and not very literate with computers so easy language would be greatly appreciated.

Posted by Isobel M
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When running strictly on battery power, the notebook's power manager will reduce the back-lighting (the brightness) on the screen in order to lengthen the battery time / run time. Plugging it in, the manager sees the full power and increases the back-lighting to full since it will not discharge the battery (the manager will be actually charging the battery at the time as well).

You can shut the hibernation mode off if you like. If you want to check the settings for this, right click on an empty spo
t on you computer's desktop, and in the drop-down menu, left click on properties. A window with several tabs will open. Left click on the screen saver tab. Near the bottom, you will see a button that says 'power'. Click on that and it will open the power options window. There will be a tab on top that says hibernate. Click on it and there will be a check in the box to enable hibernation. If this is checked and the notebook is running on battery power, if it sits long enough it will sense the battery is getting low, after a while it will start the shutdown process on you computer. Just uncheck the hibernation box to prevent your notebook from going into hibernation.

At the bottom, click 'apply', then 'okay" and your notebook should not go into hibernation again.

I have notice this on several models of Dell computers. If you go out to the Dell support site, you may find a repair software patch for the power manager. Sometimes, even when hibernation is turned off, the machines will go to sleep after a time when they are inactive (even when plugged in) and end up going into the shut down cycle you described. Check at the Dell website for your particular machine to see if they have a software fix for this.

(You can also find the power options menu by going to start, settings, control panel and the power options will be in there as well.)

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