Help me choose battery/pack? - Page 1
Forum section: electronics / projects

author: newastrocity
author_rank: Contributor
timestamp: February 24, 2015, 01:03:36 am
content: Hi guys,I am working on a project, my box is working great using a Lenovo 24v AC adaptor, and I'd like to make my box nice and wireless. The operating range for the devices contained in the box can run from 20-30v, and the current draw should be between 450-500mA. I'd like to power it for at least 6 hours, so need around 3 amps, and I'd like to do it cheaply but also in an idiot proof way (I will be giving it to someone, so it needs to be easy to recharge the batteries).I went to a couple stores and they don't have Lithium Ion rechargeables, only NimH. Some of them have 9v NimH's for around 9USD a piece. But they only provide around 200mA, so I'd have to put 3 in series to get 27V and then add more batts in parallel to get the amperage. With LiOn I'd need a protection circuit as well.I can easily pick up this DC DC converter cheap and run off of 12V to get my 24v:http://www.vetco.net/catalog/product_info.php?products_id=15033&keywords=dc%20dc%20converterWould it be easier to get 12V and use this? to calculate my amperage I will get, do I just use the amperage of the batteries and divide by around 92% to get how long it will run? Or should I get individual Lion cells?Any advice for a cost effective way to get 20-30V in a safe manner would be most helpful. Lastly, what is the best (and also easiest) way to handle switching between the AC adaptor and the battery pack, like if they were plugged in at the same time? I mean for safety or not wasting the batteries if the AC adaptor is plugged in. I won't always be there to supervise the useage of the box, and I'd like to make it idiot proof. It would be great if I could keep the battery setup inside the box and just have a cord to recharge it.Thanks fellas!(edit: I have already looked around at various battery prices, and I understand I will have to shell out some money, these things aren't cheap, just looking for safety tips and any money saving tips, I am willing to put in the work, is this possible to do for under say, $60?)

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 24, 2015, 01:26:35 am
content: Quote from: newastrocity on February 24, 2015, 01:03:36 amThe page says:QuoteWith a 3V input you could just use a hunky LiPo/LiIo 2S cell, protection circuit and charger. The discharge current of the 2S cell should be about 3C to be on the safe side. All that you get at allmost every Hobby-RC shop.A single Li* cell would need 5C discharge current, charging them is simpler because you do not have to balance them when charging.But you do not have to balance a 2S cell when charging but it is better if you do.Most quality cells should have a decent life without balanced charging.But a charge/discharge protection is must have either way, but these "protection boards" are fairly common and cheap.

author: newastrocity
author_rank: Contributor
timestamp: February 24, 2015, 01:44:32 am
content: Thanks for the quick response! So I found this:http://www.batteryspace.com/polymer-li-ion-cell-3-7v-5000-mah-6767100-2c-18-5wh-5a-rate-ul-listed-un-approved-ndgr.aspxWould I just need this batt (it has 5amps which is plenty!) and the DC-DC to get the voltage up, and then a protection circuit? I hope this somewhat stupid question makes sense: When upstepping such a big jump from 3v to 20+volts, is the downside higher current draw on the battery? I mean, I assume the extra voltage comes at a cost. In other words, is it right that the less voltage I step up, the more efficient the battery will be?Are all Lithium Ion protection circuits pretty much the same, or do I need on geared for that battery?And last question for now, any idea on the best way to handle switching between the batt and the AC adaptor safely?

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 24, 2015, 04:51:47 pm
content: Quote from: newastrocity on February 24, 2015, 01:44:32 amYes and no, 5Ah is not nearly enough to power your circuit for 6 hours, because...QuoteThe efficiency is (at first) not the primary concern, you guessed right that the "extra voltage" comes at a cost.Lets start with a few know values...24V current draw will be around 500mA, lets make that 600mA to be on the save side.Thats 14,4W power consumption on the 24V rail at 600mA (P=U*i)Now we calculate the the current draw from the battery.The battery voltage will start at 4.2V and discharge to 3,most of the time the voltage will be around 3.7V.Since the highest current draw will be at lower voltages we will use 3V as the battery voltage.We know that we need 14.4W from 3V, that will be a current draw of 4.8A at 3V (I=P/U)That means that your 5Ah battery will be depleted in unter 60 Minutes.The 4.8Ah needet is less then 5Ah, wow you may ask, why in unter 60 Minutes?That is because we can not deplete the battery fully, there will be a bit charge left in the Battery. The discharge end voltage of many modern LiPo is around 2.5V but you should not discharge them that low.Since the DC-DC converter will only go as low as 3V we will not have to worry about that annyway.The next thing is, efficiency of the converter.The converter will "waste" about 10% to 20% of the power, yes it says "up to 93% efficiency" but only under very specific conditions this number will be met.That means two things, we need more dischrage current, the 4.8A will not be enough and it will run not as long as we have calculated.Lets make that an even 5A of discharge current.And lets say that we will only get 75% of the rated capacity out of the battery.Putting all these numbers together you would need a battery that has at least 37.5Ah capacity (5Ah * 6 hours + 25%) .QuoteIn this case, you would need a adjustable "LiPo saver", that is a little device that plugs in between the LiPo and the device and cuts of the battery at a set voltage.3V in this case, or maybe 3.1V to give it a bit of safety margin.Oh damn... we got a problem!I just had a look at the DC-DC converter, it has a maximum input current of 4A wich is too much since we will need 5A.You will need a different DC-DC converter, or battery.I would opt for a different battery, a LiPo battery with two celles wired in series.That is a 7.4V 2S battery.That means a few calculations we dit above gety cut in halve, since we have double the voltage.14.4W on 24V at 600mA2.4A current draw at 6V18.75Ah battery capacityThat meany you would need a balanced charger for the LiPo.QuoteSwitching between battery and AC adapter could be done in various ways, the by far most simple would be a simple switch.

author: picandmix
author_rank: Frequent Contributor
timestamp: February 24, 2015, 04:54:28 pm
content: Its all about the Watts ..Your battery is 3.7v x 5 Amp Hour = 18.5WHour as stated in the datasheetThe desired voltages is say 24v x 0.5A Hour = 12W Hour but add in for the Converters efficiency lets say its 15W Hour.So for every hour you need to run you need one of those batteries, though as they are used there lowering efficiency means less time.As for switching between your 24AC supply, as long as its resultant DC voltage is higher than the battery/converter then you can simply run it in a diode OR way - see picWhile I have done little with buck boost converters I am told that at currents nearing theirmaximum and for long periods like you intend to use, they will need additional heatsinking and /or cooling.Being as the device you list is close to your desired wattage you might want to look for a bigger unit so its easier to cool .