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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
IMO a very simple system can be implemented, for various reasons:
A suitable number of series solar cells connect via a diode to the required (e.g. 3) number of lithium cells, no charge regulation is required as the maximum charging current is determined by light level, and the voltage is 'clamped' by the cells/battery. The esc (with under-voltage inhibit) can connect directly to the battery. Over the time frame and number of charge/recharge cycles likely, cell imbalance can be ignored, provided that all cells start with similar charge levels.
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Lithium cell voltage drop during discharge is somewhat load dependent, but many LiIon/LiPol managers simply ignore the <40% depleted battery Constant Current mode part of the charging cycle.
In general, as the ESC drivers are running directly off the charger-bypass/current-protection circuit (3 or 4 cells), they can avoid the cost/weight of power conditioning that can only make it less efficient. Thus, the energy conversion rating is good if the parts are sized appropriately to keep the dissipated energy losses to a minimum.
Has anyone tried a split power rail system before... like 2 latching relays?
Mode 1: MPPT Charging cycle i Charging or Resting 1/2 capacity bank A (3S 1.2Ah) Using 1/2 capacity bank B (3S 1.2Ah) Bank B Maintaining control logic capacity for bank C (A/B step-down to C with 2S 600mAh)
Mode 2: MPPT Charging cycle ii Using 1/2 capacity bank A Charging or Resting 1/2 capacity bank B Bank A Maintaining control logic capacity for bank C
Mode 3: Performance climb Using 1/2 capacity bank A Using 1/2 capacity bank B Bank A+B Maintaining control logic capacity for bank C
Mode 3: Glide mode Maintaining 1/2 capacity bank A Maintaining 1/2 capacity bank B Bank A+B Maintaining control logic capacity for bank C
Mode 4: Dump mode Toggle between Mode 1 and Mode 2 every 5 minutes. Bank A+B Maintaining control logic capacity for bank C
If you want to build something that stays aloft a longtime, than you need to restrict your MPPT charge managers to maintain 1 C (and size your craft around these LiPol properties). In this example, that is 1200mAh * 1Cbatt = 1.2A charge rate at the battery, and thus very inexpensive to find. The solar array should simply be wired in such a way where the charging voltages are always above the minimum charger input voltages, as most are setup in a buck configuration (a 24v to 12v drop is probably >86% efficient).
Recall that LiPol have a Constant Current and Constant Voltage mode in a charging cycle, and will remain in the Constant Voltage mode if the bank never drops below 66% capacity. Thus, ideal for variable solar power input for both the MPPT chargers, and maybe a way to get around that 1 Cbatt charge rate limit.
For bank C, which is only really used to power logic during switch over and brown-outs... it is simpler as the fully charged primary bank A/B should always meet the input requirements:
To stay running, lets say that: Given a 1.3m wing Skywalker Falcon can manage over 30 minutes with a 2.2Ah 3S Cell... Each bank would need to be rated to supply minimally 2.2Ah per hour, as the solar charger would need to be able to maintain the 2.2Ah average top-off charge rate (in CV) while the other bank was being depleted. Thus, the air frame relative to the above would need to be able to manage >1.4kg loads (a large 3m wing span), and only run a 800-900kv undersized motor on average.
I am sure if you play around with the numbers there may be a fair compromise someplace, but I suspect going larger actually only makes the problem more difficult.
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