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create stable voltage
#1
hey everyone,

my last project went rather successful, converting my dynamo driven bike lights to lithium powered. Because of the rather small current draw, I was able to get away with using a DC-DC converter to get a stable voltage for the lights.

my next project is a bit more powerful. I intend to build a big mobile battery for my sailing kayak.
The battery has to support the following items: 2Blue Robotics thrusters (25A each 6-20V I would like around 14-15 V max), lights, 5 cigarette lighter outlets(10 A each, 12V) and a bilge pump.
I calculated that I almost need 2kwh for the range I want to get eventually. so far so good.
The problem occurs in the pack voltage. I need at least 12 V for the cigarette outlets to work properly. so if I drain my cells to 2.8 volts I need at least  4-5s. but then charged the voltage is too high for the 12 v equipment I believe the maximum voltage is around 14-15 v but at 4.1 v per cell, the pack has 20+ volts
the thrusters are ok with up to 20 v so if I only charge to 4 v per cell everything should be ok. the other instruments cannot handle the voltage so I thought of dc-dc converters but I need rather many of them to supply all my other devices and they all have to provide at least 10 A (max amperage from cigarette lighter outlet I will never use 10 A but I do not want to ).
As an encasing, I want to use a big plastic Ammunition Box or something equal but it has to be watertight because my kayak ist open an often waves get into the boat so there sometimes are 4-10 cm of water in the boat if there is no time to bale. Maybe I will add a bilge pump. Because of the plastic box, I don't think I can get all my dc-dc converters in there because of the generated heat.
is there a better solution instead of the DC-DC converters

the other equipment BMS and charger I still have to research a bit.

thanks,
Albach
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#2
I would recommend forgoing 12/24V, and go with something else. The thrusters can do 20V. So, I would say go with 5s, which gives you 16V - 20.5V (3.2V - 4.1V). Then use buck converter for the 12V cig ports. The reason for the 5s is to help lower amp draw.

Or, you could go with a higher Xs series, then build your own buck converter to handle the higher amps needed. This may seem a bit daunting, but they are fairly simple. A few FETs, some diodes, capacitors and a timer, you're good to go. You might even be able to use a 555 timer instead of an MCU like an arduino if you don't want to program anything. Then use a variable resistor (trim pot) to adjust the timing pulses of the FETs.
Julian Ilett has built his MUPPET which is a buck/boost converter. Even though his uses a 1 of each of the aforementioned parts, it should be easily scaled up.
Or use a PWM to change to a somewhat AC current, pass through a transformer, then re-rectify it on the other side.

The advantages of going higher series is less amps draw on the cells, meaning longer life/duration. Unless you go with something like LiFePO4's or other high drain cells, cycle life, and capacity, will be shortened.
I bring this up because you can't exactly put a 7s40p in the kayak. Would kinda make it a bit heavy and/or bulky.
You might even look at headways instead, too.
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#3
DC-DC converters that use switching technology inside tend to run fairly cool vs linear regulator ones.
My go-to for converters is Daygreen on AliExpress https://daygreen.aliexpress.com/store/1487825
Most of theirs are potted in epoxy & "waterproof" with heatsink fins.
IMHO they're not quite as easy as Korishan suggests unless you're into electronics!
Running off solar, DIY & electronics fan :-)
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#4
I'd go for a 4s LiFePO battery. With that you don't need any DC-DC converters and you can run everything from 12V. How much space do you have available and is there any weight limit?
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#5
you can use a voltage stabilizer to have  stable 12 volts, its what I use to protect my fans from voltage surges. Input voltage is 12-24 volts, output is 12 volts 10 amps, you can find on ebay for less then 30 dollars. Its waterproof. 

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#6
Well, no matter how you want to call it, this is a DC-DC converter which brings us right back to the beginning Wink
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#7
Thank you for your answers
Volume or mass is not a big problem because the kayak seats 3 and I only can use 2 seats because of the sailing rig maybe I can get my hands on some 24v equipment for 18 wheelers (5v USB charger, maybe also the bilge pump). Lighting will be 12v but also low amperage.  I do not want to do daisy-chain multiple converters on to another and use hi amperage devices ( bilge pump and thrusters ) without DcDc converters so I will also look in lifepo4.
thank you for your suggestions i will take your considerations in acoount.

albach
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#8
That's perfect, the one reason for not using LiFePO cells is usually size and/or weight because their energy density is a bit lower than what other lithium cells can offer. I'd suggest cylindrical Headway cells (40152, 15Ah) or prismatical Winston cells (40Ah and more). The Winston cells are probably better in your case because you need at least 160Ah at 12V to get the needed 2kWh. Four of their 200Ah cells sounds reasonable. Total size and weight would be 222x256x362mm and 32kg.
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#9
I like the idea of Winston cells.
They seem good online and a lot less welding work as the cemended4s 50p 18650 pack
Furthermore, I can completely remove all DcDc converters and have rather efficient power transfer. the cost is the pack seems to be a bit more efficient than the 18650 if you do not harvest them but buy them in bulk.
I will not forget the Headway cells they may fit on some other project.
I will work in a planned schematic for the battery an post it tomorrow.
Thanks for your help
albach


PS can you recommend any good literature for Lipo4 cells (charging balancing etc.)

@Dark raven Schöne Grüsse in den Norden
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#10
Grüsse zurück! Smile

LiFePO4 cells don't have special requirements. They work like any other lithium cells, just with lower voltage.
You just have to follow the specifications, ev-power.eu has put everything you need to know into the description of the cells and also provide a datasheet: https://www.ev-power.eu/Winston-40Ah-200...html?cur=1
They also sell these cells bundled with connectors and a BMS: https://www.ev-power.eu/Winston-40Ah-200...html?cur=1
I have no idea though what the BMS can do and cannot do, don't know if it's any good. If you decide to get single cells then you can find the Winston cell connectors listed under accessories.

I've never bought anything from them, but they seem to be the main importer of these cells in Europe. They are based in the Czech Republic and the prices don't include VAT yet so 21% would be added to the stated prices.
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