To lithium LifePo4 or to not Lithium LifePo4 on a sailboat (LFP part 2) what do we need ?
warning : another boring techy post , only mildly interesting for other sailors and boaters that are contemplating a switch to LifePo4 batteries, but there will be another video soon , I think 😉
Even though I am still in the planning stages on which exact battery , or cells , to buy and get shipped, there are a lot of things that can already be decided and need to be done or at least planned for and thought out.
Charging with sources not specifically made or suitable for LFP
My biggest worry currently is that we have several charge sources that are not designed for charging LFP. On Blue Pearl those sources are
• 2 alternators (1x 120A and 1x 70A) on our main engine
• 3 flexible solar panels (2x 30Wp and 1x40Wp) on deck with PWM controllers
• An old(er) 40A Dolphin Battery charger
• 250W Wind generator
These charge sources are great for Lead based batteries but as I stated before, LFP does not like to be charged to the same level and voltage as lead based batteries and severe damage to the LFP batteries can occur if you still do this .
As I never trusted the large alternator (I think this alternator , with external sterling regulator, contributed to the early demise of our current AGM battery bank so its been switched off for the last few months) my initial thought was to still use 2x AGM batteries for the starter and bow truster but completely separate the alternators from the LFP bank. This would certainly simplify the installation but also cut us off from a potential 170A charge source .
My 2nd idea was to install a battery switch between the AGM bank, which would be charged by the alternators , and the LFP bank. So that if the need arose I could ‘flip the switch’ and charge the LFP bank from the alternators if/when needed. This would require extreme caution and minute monitoring as the LFP bank can be overcharged in minutes when it nears the ‘fully charged’ phase . With this set-up I originally planned to put the wind generator and PWM solar controllers directly on the LFP bank , thinking the charge current they provide is not big enough (between 6A and 20A combined) as they match our use on board. But there is still a tiny chance that even this small of a charge current at higher (>14 volt) can and will damage the LFP bank if this charge current if held too long .And then there still is the 2nd battery charger that outputs 40A when on shore power or when the generator is running , but has no LFP setting , so again , it can damage the LFP bank .
The 3rd idea was to add some sort of a ‘buck / boost’ DC/DC converter between the AGM bank and the LFP bank , this would in effect limit the output voltage of whatever is connected to it , tuning the voltage down or up to suit the LFP bank. If the alternators voltage output is 14.5 volt , the DC/DC converter tunes it down to 14 volt (or 13.8) so the LFP bank would still get its charge current and at an acceptable charge voltage !
After a lot of searching I think I have decided to go for a Sterling ‘Battery to Battery charger’
that seems to be just right for charging an LFP bank. And also it will allow me to put everything that is not specifically suited for charging LFP on the AGM bank and the Sterling will make sure that the voltage never rises to a point where damage can occur to the LFP bank
The ‘downside’ (or upside?) of this option is that the charge current is limited to 60A , for the 40A Dolphin battery charger , the wind generator and flex solar panels this is not an issue , as even combined they will hardly ever reach the 60A current . The alternators however are rated for 120 + 70 = 190A , so I am ‘loosing’ 130A , or am I ?
The maximum rating of the alternators is just that , a ‘maximum’ rating , and most alternators (including these) are not built to output that maximum for very long ! When you try to do this , and this is very easily done with LFP as they will draw any and every amp made available to them, your alternators will go up in smoke due to overheating very quickly which has already happened twice with our alternators charging a quite empty 500Ah AGM bank 🙁
So by limiting the charge current to 60A I am actually saving my alternators and providing the LFP bank the charge current and voltage they need . And I might still use a battery switch between the Alternators + AGM’s and LFP bank , just to make sure I can cut off that charge source if/when needed and connect the wind generator, flexible solar panels and 40A charger directly to the Sterling , so they are always part of the charging process.
Protecting the LFP bank from overcharge and undercharge
To further protect our LFP bank we need a BMS (Battery management system) that will do a few things :
• Keep all cells ‘in line’ while charging , making sure they are all charged to the exact same voltage
• Protect the LFP bank from over charging and taking them too low
I am going to achieve this by letting the BMS control some big ass Blue Sea RBS ML-7700 relays ,
these relays are true ‘latching relays’ so when they are ‘set’ to on , they don’t draw any power anymore. The problem here is that most (almost all !) BMS manufacturers use ‘normal’ relays to disconnect the LFP in case of a problem and these ‘normal’ relays consume power while being activated. So after a lot of searching I finally found manufacturer in Australia (EV Power) that is , as far as I know, the only one that makes a BMS that actually can operate a latching relay , (yes they can operate a latching relay but not the BlueSea 7700 : see part 4 for solution) but this manufacturer does not have the ‘ohhh pretty !!!!’ Display or anything like that , its basic , simple, gets the job done , intelligent solution without too many bells and whistles and as stated in a previous post we still have our Simarine PICO battery monitor with a Wonderfull display so I will use that to fulfill my inner gadget freak and will probably forgo on the display that EV Power Australia offers . The Simarine PICO also has several settings for audible alarms , which I will use to alert me before the actual BMS (last resort) protection kicks in.
Btw , Simarine ? If you ever read this , I am quite (read REALLY !! ) annoyed with your lack of technical customer support !! I was really impressed with the support when I bought the system but ever since that its been a steep way down I’ve send you several emails asking how the relay output works on your SCQ25T but I a still waiting for an answer , it would be great to use these outputs as an alternative or addition to a BMS , but dead silence .Ok.. enough complaining , I should have donated my money to Victron while refitting Blue Pearl and would have gotten great customer support and an active technical forum to ask questions
Simarine just answered my question , their alarm output can be used !
So .. enough technical ramblings .. lets have a Rum on Beautiful Bonaire and see what we are actually going to order and how we’re going to get everything to where we are !! Which is , in itself , quite challenging
btw , if you made it this far I applaud you and I am just about to hit the ‘send’ button on a money transfer to a Chinese company on Alibaba I have never dealt with before to send them 2.5 boat bucks hoping they will fulfill their promise and send some LFP batteries
to a freight forwarder, in a country where we will never go to , who promises to send the batteries to a Marina we’ve never been to before, or even contacted, in a country we have yet to visit !!!
Welcome to the wondrous world of cruising and wish me luck !