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Re: MPPT V's PWM regulators

Posted: Tue Aug 20, 2013 4:50 pm
by Trev
wont the Batteries own BMS disconnect the batteries from the charge when they have had enough??

Geez I am out of my comfort zone here.

Re: MPPT V's PWM regulators

Posted: Tue Aug 20, 2013 5:28 pm
by groper
Yes, thats the idea of a BMS... thought i said that? :D

a BMS can be a complex peice of kit that can do many things, but all it really needs to do in order to keep a lifepo4 battery healthy, is keep the voltage of the cells, above a minimum point, and below a maximum point - thats it! The only other thing it should do, is keep the cells within the battery, balanced. A BMS really doesnt have any other function.

What i said earlier was, that i will not be running a BMS... instead, i prefer to set things up so that a BMS is not required by limiting the voltages provided by the charging, and a simple isolating cutoff at my 20%DoD setpoint. Periodically, say once a month depending on usage etc, i will manually check the individual cell voltages with a multi meter, and balance them with an individual cell charger for $50... like this one -> http://lithbattoz.com.au/index.php?page ... l-chargers


Most BMS vary considerably, youd really have to look closely at how they are setup or how programmable they are. Ive noticed on some solar regulators designed for lithium batteries like the genasun, the float voltage is a bit too high for best longevity @ 14.2V - it should be more like 13.6V - who cares about the last 5% of charge if the battery will last 3 times as long???

I also think that the low voltage cutoff on most BMS, is set too low for the same reason, reduced longevity. If you stay between 20%DoD and 95% DoD and sized your bank accordingly, the battery will probably last longer than we will - should be 3000-5000 cycles which is 10 years assuming a full 80% cycle everyday - which isnt going to be the case. Depends on how much you use the boat etc... Have you seen the quoted lifespans by the manufacturers? You will notice that most of them quote 5000cycles for 70%DoD and 3000cycles for 80%DoD. If you cycle them deeper than this, lets say 90-100%DoD then you wont get anywhere near the 3000cycles they quote. It drops off sharply to something like 750-1000cycles - so now your back to only a few years usage... very important consideration on that point.

BMS`s are not fail safe either - there are many reports of failed BMS on the web, and a failed BMS can stuff your battery... so if there is nothing to fail, isnt it better than needing something NOT to fail? :mrgreen:

What BMS, battery, and solar system are you looking at anyway?

Re: MPPT V's PWM regulators

Posted: Tue Aug 20, 2013 9:03 pm
by mahnamahna
This is a fantastic thread. One of the best for a long time.

I intend to deal with the over drain with one of these:

http://www.ebay.com.au/itm/170818923742 ... 1423.l2649


attached to a relay/solenoid cut out that opens the circuit at my predetermined voltage or AH's either way will work, if I leave 20% in the bank I am safe. I guess, if I had an elec start genset it could also auto start the gennie.

I initially wanted one of these just so I could periodically check my house power levels (ah's) and ensure the voltage is still healthy, but the ability to run a relay too is handy.

Re: MPPT V's PWM regulators

Posted: Wed Aug 21, 2013 8:34 am
by Trev
groper wrote:Yes, thats the idea of a BMS... thought i said that?
What I mean't was, If you had BMS then would you have the same issue with your alternators?
Trev wrote:What BMS, battery, and solar system are you looking at anyway?
AT this moment in Time I am having the following
520w of 12v solar
Plasmatronics "40a Dingo" solar reg
400a? lifepo4
2 x 80 alternators
and a Honda portable genie

I see the panels and the batteries as being expandable at a latter stage if needed, and if I were in need of a wind generator down the track I would go with the pommy Ampair.

Trev

Re: MPPT V's PWM regulators

Posted: Wed Aug 21, 2013 10:35 am
by groper
Well yes... your BMS *should* take care of it - but you need to check whats its capabilities are as they are all different.

Best i can figure is BMS`s in general, were plugged as so important for teh lifepo4 batteries mostly because they were being sold into a market thats completely setup for lead acid batteries. In order to cope and prevent mass failures, the sellers needed to sell the importance of a BMS so that they could be a "drop in" replacement for lead acid in the existing market. So the BMS needs to cut off the charge from the alternators once its full, as the all existing alternators across the board are designed for lead acid and have the charge voltage set too high and will cook the lifepo4 batteries.

To me, building from scratch, this seems like a band aid approach. Rather than rely on a BMS - which can fail and is costly to buy in the first place - i thought it better to change the charge voltages and solve the cause of the problem. So im looking at a means to limit the voltages from the alternators via a custom reg or perhaps a simple DC-DC converter will handle it - i havnt looked into it yet as i have more important things to do atm - like get it to paint before the next wet season.

If you expand your array above 520W - that controller your looking at wont be able to cope - its limit is 40amps. So lets say you added another 260w - youd then have 780W - which is what i plan on running as i only have 27a alternators on these outboard motors.

So if you expand to 780W @ 13v, your panels will be able to supply 60amps - 13v*60a=780va. Your controller will dump 1/3 of its capacity and youll have to upgrade or buy a second unit if you expland your array... im buying a 60a controller straight up.

400ah of lifepo4 battery should be fine i think - im going to try and get away with 300ah to save a bit of weight and cost. The 780W of panels should have no worries fully charging from empty in a single day... The economics of it these days, means its cheaper to have a larger array and smallest battery possible - provided you can stay within the 80%DoD. I figure i only need enough energy to get me through 1 night as the array has the capacity to fully charge in a single day. If the weather is bad, then the engine alternators can be run and also a gennie as a 3rd backup. Good thing is, a 400ah lifepo4 pack, could safely take 400amps of charge current - so if you had teh capacity to generate 400amps, you could fully charge your bank in less than an hour! try that with lead acid...

My energy needs will be pretty small, as ill run gas cooking and hot water etc... Your milage may vary depending on your energy needs.

Re: MPPT V's PWM regulators

Posted: Wed Aug 21, 2013 1:39 pm
by Whimsical
groper wrote: Best i can figure is BMS`s in general, were plugged as so important for teh lifepo4 batteries mostly because they were being sold into a market thats completely setup for lead acid batteries.
Actually BMS's came about from the electric car and bike sectors as they use high voltage strings and fast discharge rates. If u have a lot of cells in series and a per cell charging voltage just a little over the min of 3.4 then once one cell becomes full it will have the little bits over 3.4 times the number of cells applied to the full cell. EG 24 cells times .2v + 3.4 = dead cell very fast. The car guys also use a higher per cell charge voltage to speed up the recharge time.
We only have 4 cells so if the charge voltage is kept low and 1 cell does charge a bit ahead of the others then no great problem as there is not enough voltage applied to the charged cell to do much damage. We also use a very low discharge rate and that helps a lot in keeping the cells in balance.

Re: MPPT V's PWM regulators

Posted: Wed Aug 21, 2013 2:06 pm
by groper
Totally agree whimsical... just that even for our house bank low charge/discharge rates, the suppliers all vehemently tell you that YOU MUST have a BMS for a lifepo4 battery OR ELSE! - its almost like a scare mafia campaign... The BMS is a very simple bit of circuitry which isnt worth more than a couple of dollars. But theyll try to jam you big dollars for one as it "protects your investment"... Now im not saying this isnt true, but the margin in the BMS is clearly 10 times what it is for the battery - so this is where they get their money.

And for the reasons i stated earlier, you cant just drop in a lithium battery without a BMS into a system designed for Lead acid or it just wont last like it should thanks to the charge voltages of typical engine alternators and other chargers etc

Theres some guys whove been running lifepo4 battery systems in their RV`s for quite some time and they say for teh low charge/discharge rates, even after months of use, the cells are still balanced - so the need to balance them is pretty rare in house bank type applications - which is why i dont mind doing it manually as it will most likely only be an annual event...

Re: MPPT V's PWM regulators

Posted: Wed Aug 21, 2013 3:28 pm
by 44c
groper wrote:I also plan on running lifepo4 batteries.... these will be primarily charged by the solar system - im not sure how im going to go connecting the engine alternators to the house bank as it require a custom regulator to be done safely - so i may just have a stand alone solar battery system.
If/when I eventually end up with Lithium batteries, I'm thinking of having the motors connected to a dedicated lead acid start battery, which can then charge the house lithiums via a DC-DC converter. Just a thought.

Re: MPPT V's PWM regulators

Posted: Tue Jun 17, 2014 3:31 pm
by dennisail
I had a long think about this and chose PWM, which is still a very good option.

Here is mashup of a response I wrote on a another forum. I have a professional electrical background, not that it really matters. I usually ignore the "card" when others present it :p

I got 800W of panels (4 x 200W) from low energy developments on special for $880 delivered. The panels come with an alloy frame but they are very light. I think around 50kg for all 800w? $1 per 1W seems the going rate for a good deal.

I believe the output of the framed ones stays higher when it gets hot as they can dissipate heat from underneath easier.

After careful consideration I just purchased a $75pwm solar controller which seems to work well and has enough adjustability. I wired each panel with 6mm wire. In the next couple of years I think I will be able to buy cheap MPPT controllers for each panel. I didn't think the expensive MPPT controller was worth it as they cost as much as another 800W worth of panels!

I have seen up to 45A coming from this setup and I am never short of power. Thinking of getting a convection microwave oven when I get my lithium bank

http://www.ebay.com.au/itm/New-60A-Sola ... e47&_uhb=1

I run each panel straight to the controller. The bigger the wire the better! This matters even MORE with mppt as you want the voltage to be as high as possible to get the best results from the voltage/power conversion.

I done this firstly to limit voltage drop (not as important with PWM as your battery is say 14.5V and the panels max voltage potential is up around 18V or so, so that extra voltage wont be used to produce much more power anyway with PWM). But the fact I do have one cable per panel means I can connect individual MPPT controllers easily when they are cheap enough. Get back to basics here. The whole point of MPPT is to find the best operational point for each panel based on environmental conditions, which can not happen on a boat with shading when all panels are hooked to one controller!


There are already many cheap MPPT units out there, and I suspect some may work well right now, but without doing the testing you will be gambling. I think given some time, what is cheap and works will become apparent. And in the meantime my $75 controller works very well.

When I say I used 6mm wire, I mean that stupid automotive size scale which I do not like. I think the core is something like 4.6mm2, or 11AWG. The max run is something like 6m and the shortest 3m. I would have liked to use something bigger but that is just what was handy at the time.

Use this calc here. If you are planning on using MPPT one day, see if you can size it for not much over 2% loss.

http://www.calculator.net/voltage-drop-calculator.html

There are so many debates on the net regarding MPPT etc. With so much utter horseshit on both sides of the story.

Anyway, you can run panels in series with MPPT which means smaller wires. But there is a greater loss in the voltage conversion the bigger the stepdown. Do not forget that the DC - DC conversion does not happen for free. The supposed margins for MPPT are pretty small, so voltage conversion losses can not be ignored, the bigger the step down the more loss! Add in diode losses for shaded panels in series, and hot panels with low voltage, then how much gain is left?

That is the real issue when used on a boat, that if groups of panels are connected to one MPPT controller it is impossible for the controller to find the best operating voltage for each panel if one panel is shaded, which is VERY likely on most boats. Therefore the whole concept will work best with one controller per panel IMO. And for that to be worth doing the controllers need to be cheap. I could fit 1200w of solar panels on my bimini alone and they are cheap. Now Groper is saying they cost $190 for 250w panels!

In conclusion, MPPT has potential but one needs to be realistic about the gains VS cost spent in other areas of the system. One needs a good understanding about exactly how MPPT works to deliver gains when designing the system.

So which cheap Ebay MPPT controllers actually work then?

Re: MPPT V's PWM regulators

Posted: Tue Jun 17, 2014 4:30 pm
by dennisail
mahnamahna wrote:This is a fantastic thread. One of the best for a long time.

I intend to deal with the over drain with one of these:

http://www.ebay.com.au/itm/170818923742 ... 1423.l2649


attached to a relay/solenoid cut out that opens the circuit at my predetermined voltage or AH's either way will work, if I leave 20% in the bank I am safe. I guess, if I had an elec start genset it could also auto start the gennie.

I initially wanted one of these just so I could periodically check my house power levels (ah's) and ensure the voltage is still healthy, but the ability to run a relay too is handy.
Awesome find! Seems this is all the "BMS" I will need :) My $60 PWM controller has a programmable low voltage disconnect that could be used to cut a relay to the bank. It also has a removable temp compensation probe, and a max bank charge voltage setting so it looks promising for use with a lithium bank.

As mentioned, all you need to keep a lithium bank healthy is to keep voltages between 2 safe high and low points. And for low loads like a house bank, periodic manual monitoring of cell balance is all that is required. A few of these will make it easy for less than the cost of a pint of beer.

http://www.ebay.com.au/itm/1-3Pcs-0-32V ... 909&_uhb=1


What is the best place to buy lithium batteries ATM? My current bank is fully stuffed. I was thinking of buying one cheap AGM to keep me going until I cruise in 2 years, in the hope lithium will be cheaper / better by then. Should I hold off or just go lithium now?